# USMLE Step 1 Free 120: Item Map and Explanations

> What is the Step 1 Free 120?

The Free 120 is the set of USMLE Step 1 sample test questions the NBME and FSMB publish for free on usmle.org. The current booklet, updated April 2026, holds 119 items in six blocks with an answer key, and a matching interactive version adds one audio item you cannot get from the PDF.

## Where to get it

- Official sample questions page: https://www.usmle.org/exam-resources/step-1-materials/step-1-sample-test-questions
- Official PDF: https://www.usmle.org/sites/default/files/2021-10/Step_1_Sample_Items.pdf
- Official interactive testing experience: https://www.starttest.com/custom/usmle/?stpf1

Built from the Step 1 sample-items booklet marked "updated April 2026", the PDF linked from the official Step 1 sample-questions page. StepGenie does not host or mirror the booklet, and does not reproduce its vignettes, answer options or images.

## How the booklet is built

- The booklet carries 119 numbered items and prints the answer key at the end.
- The items are laid out in six blocks: five blocks of 20 items and one of 19.
- One block is short because a multimedia item with audio findings exists only in the interactive testing experience, not in the PDF.
- The booklet gives 28 minutes 30 seconds for that shortened block, against 30 minutes for a full 20-item block -- the same pacing as the Step 1 format effective May 14, 2026.

## Item map

One row per numbered item in the official booklet. block = the booklet block the item sits in. system and topic are our own editorial tags. paraphrase is an original one-line summary of what the item asks, written to be substantively different from the booklet text. No vignette text, answer options or answer key is stored here.

### Block 1

| # | System | Topic | What the item asks | Explanation |
|---|---|---|---|---|
| 1 | Blood & Lymphoreticular / Cardiovascular | Inherited thrombophilia / pulmonary embolism | Young woman post long flight with hypoxia, tachycardia, unilateral calf swelling; most likely underlying cause. | published |
| 2 | Musculoskeletal / Immune | Rheumatoid arthritis serology | Middle-aged woman with years of hand stiffness and deformities (photo); which abnormality confirms diagnosis. | published |
| 3 | Nervous System | Pontine stroke localization | Elderly stroke patient with left abduction failure, right hemiparesis, right sensory loss; predict an additional finding. | published |
| 4 | Multisystem / Infectious | Acute HIV infection | Man with fever, pharyngitis, generalized lymphadenopathy, pancytopenia, negative heterophile test; most likely diagnosis. | in review |
| 5 | Renal / Urinary | Drug-induced acute interstitial nephritis | Woman on prolonged IV oxacillin develops fever, rash, eosinophilia, AKI with urine eosinophils and WBC casts. | in review |
| 6 | Nervous System | Syringomyelia etiology | Young woman with progressive arm weakness and cape-like sensory loss; cervical syrinx on MRI; which history to ask. | in review |
| 7 | Cardiovascular | Peripheral arterial disease pathogenesis | Diabetic smoker with claudication, hairless atrophic cool legs, absent pedal pulses; mechanism of symptoms. | in review |
| 8 | Social Sciences / Communication | Motivational interviewing | COPD smoker not ready to quit while wife complains; choose the best physician statement. | in review |
| 9 | Endocrine / Nutrition (Pediatrics) | Failure to thrive | Toddler drinking mostly formula with weight falling off curve while length preserved; cause of growth pattern. | in review |
| 10 | Biostatistics / Epidemiology | Measurement validity | Cohort study adds a missing food item to a dietary questionnaire; which study property improves. | in review |
| 11 | Multisystem / Infectious | Chagas disease transmission | Asymptomatic immigrant from El Salvador with positive Trypanosoma cruzi screen; which vector bit her. | in review |
| 12 | Social Sciences / Communication | Patient-centered interviewing | IBS patient refusing medication returns repeatedly with pain; best physician response. | in review |
| 13 | Renal / Urinary / Pharmacology | BPH pharmacotherapy | Elderly man with nocturia and weak stream, smooth symmetric prostate; mechanism of most appropriate drug. | in review |
| 14 | Behavioral Health | Bipolar mania with psychosis | Woman stalking the president, pressured speech, grandiosity, no sleep, hallucinations; most likely diagnosis. | in review |
| 15 | Gastrointestinal / Pharmacology | H2 blocker vs PPI pharmacodynamics | Match gastric pH-over-time curves to cimetidine and omeprazole in a GERD research subject. | in review |
| 16 | Gastrointestinal / Cardiovascular anatomy | Hematogenous metastasis route | Colon cancer with new liver and lung metastases; through which vessel did tumor reach the lungs. | in review |
| 17 | Skin / Infectious | Scabies | Child with intense itching and serpiginous burrows in finger webs, waist, inner thighs; most likely diagnosis. | in review |
| 18 | Gastrointestinal / Blood | Pigment gallstones in sickle cell disease | Sickle cell teen with biliary colic after fatty meal and gallstones; underlying cause. | in review |
| 19 | Endocrine / Genetics | MEN2 / RET | Man with prior pheochromocytoma, medullary thyroid carcinoma, mother with parathyroid tumor; gene class mutated. | in review |
| 20 | Endocrine / Reproductive | Central precocious puberty | Six-year-old with vaginal bleeding, breast buds, elevated estradiol, FSH, and LH; cause. | in review |

### Block 2

| # | System | Topic | What the item asks | Explanation |
|---|---|---|---|---|
| 21 | Reproductive / Genetics | Klinefelter syndrome | Tall 16-year-old with gynecomastia, small testes, high FSH/LH, low testosterone (chart format). | in review |
| 22 | Endocrine / Pharmacology | Salt-wasting congenital adrenal hyperplasia | Newborn with ambiguous genitalia, hyponatremia, hyperkalemia, acidosis, hypoglycemia; receptor targeted by therapy. | in review |
| 23 | Renal / Urinary | Urgent dialysis indications | CKD patient with rising creatinine; which new finding mandates immediate hemodialysis. | in review |
| 24 | Cardiovascular | Acute MI on cardiac function curve | Acute STEMI with pulmonary edema and JVD; identify the shift on a labeled cardiac function graph. | in review |
| 25 | Nervous System / Pharmacology | Parkinson adjunct therapy | Parkinson patient still freezing on carbidopa-levodopa; which enzyme should an adjunct inhibit. | in review |
| 26 | Gastrointestinal / Anatomy | Left posterior thoracoabdominal trauma | Gunshot entering left posterior 10th intercostal space with shock; organ most likely holding the bullet. | in review |
| 27 | Behavioral Health / Pharmacology | Neuroleptic malignant syndrome | Psychiatric inpatient started on olanzapine develops high fever, confusion, autonomic instability; expected exam finding. | in review |
| 28 | Renal / Urinary / Oncology | Urothelial carcinoma risk factors | Heavy smoker with naphthylamine exposure and painless hematuria; renal pelvis mass; tumor type. | in review |
| 29 | Skin | Rosacea triggers | Woman with recurrent stress-triggered facial flushing and burning (photo); which additional history to obtain. | in review |
| 30 | Immune / Public Health | Herd immunity | Unvaccinated child has never had vaccine-preventable illness; explanation for good health. | in review |
| 31 | Reproductive / Immune | Rh alloimmunization prophylaxis mechanism | Rh-negative woman pregnant again after prior sensitization; mechanism of preventive pharmacotherapy. | in review |
| 32 | Musculoskeletal / Biochemistry | Gout and purine metabolism | Man with acute painful swollen great toe after lobster and beer; which metabolic pathway's intermediates accumulate. | in review |
| 33 | Social Sciences / Communication | Alcohol use screening conversation | Patient mentions a DUI citation during routine visit; best initial physician response. | in review |
| 34 | General Principles / Embryology | Lymphatic development | Child with congenital unilateral leg lymphedema from anomalous lymphatics; embryonic tissue of origin. | in review |
| 35 | Respiratory | Obesity hypoventilation compensation | Morbidly obese man with chronic hypercapnia and hypoxemia; which additional finding is most likely. | in review |
| 36 | Renal / Infectious | Hemolytic uremic syndrome | Child with bloody diarrhea then anemia, thrombocytopenia, and renal failure; causative organism. | in review |
| 37 | Cardiovascular | Hypertensive LV hypertrophy | Hypertensive woman with displaced PMI and LVH voltage on ECG; cellular process behind ventricular change. | in review |
| 38 | General Principles / Pharmacology | Carcinogen metabolic activation | Polymorphisms in activation enzymes raise lung cancer risk in benzene-exposed smokers; which enzyme. | in review |
| 39 | General Principles / Pharmacology | Clinical trial phases | New antimalarial tested for tolerability and pharmacokinetics in 80 healthy volunteers; trial phase. | in review |
| 40 | Endocrine / Biochemistry | Glucagon failure in glycogen depletion | Marathon-training type 1 diabetic unresponsive with hypoglycemia; glucagon fails to raise glucose; why. | in review |

### Block 3

| # | System | Topic | What the item asks | Explanation |
|---|---|---|---|---|
| 41 | Social Sciences / Communication | Breaking bad news | Woman with pancreatic mass and liver lesions awaiting biopsy; best next remark after disclosing the mass. | in review |
| 42 | Blood & Lymphoreticular | Polycythemia vera | Man with headaches, splenomegaly, very high hemoglobin and hematocrit; expected mutation. | in review |
| 43 | Nervous System | Temporal lobe epilepsy localization | Woman with olfactory auras, lip smacking, and emotional outbursts; choose brain region on coronal photograph. | in review |
| 44 | Behavioral Health / Pharmacology | Bulimia-related sialadenosis / dry mouth | Inpatient with bulimia and new fluoxetine has mouth pain and parotid swelling; drug mechanism to treat symptoms. | in review |
| 45 | Immune / Skin | Contact dermatitis hypersensitivity | Man with intensely itchy rash one day after clearing backyard weeds; hypersensitivity type. | in review |
| 46 | Cardiovascular / Respiratory | Chronic hypoxic pulmonary hypertension | Woman living two years at high altitude develops JVD and edema; what decreased over that period. | in review |
| 47 | Cardiovascular | Coarctation of the aorta | Child with arm hypertension, low leg pressure, and interscapular murmur; expected additional exam finding. | in review |
| 48 | Endocrine / Respiratory | Sarcoidosis hypercalcemia | Woman with hypercalcemia, suppressed PTH, hilar lymphadenopathy, interstitial infiltrates; cause of hypercalcemia. | in review |
| 49 | Gastrointestinal | Chronic pancreatitis steatorrhea | Man with recurrent pancreatitis, weight loss, oily foul stools; which enzyme replacement helps most. | in review |
| 50 | Gastrointestinal / Infectious | Amebic liver abscess therapy | Man from India with fever, RUQ pain, hypoechoic liver lesion; why paromomycin is added to metronidazole. | in review |
| 51 | Musculoskeletal / Oncology | Pediatric rhabdomyosarcoma | Toddler with painful proptosis and orbital mass; biopsy shows malignant cells with striations. | in review |
| 52 | Multisystem / Infectious | Sepsis from wound infection | Carpenter with necrotic hand puncture wound develops sepsis days later; most likely organism. | in review |
| 53 | Nervous System | Spinal cord level localization | Post-collision patient with intrinsic hand atrophy, leg spasticity, sensory loss below clavicles; lesion level. | in review |
| 54 | Respiratory | Obesity-related restrictive PFTs | Morbidly obese man with exertional dyspnea and clear lungs; predict FEV1/FVC/TLC/DLCO pattern. | in review |
| 55 | Reproductive | Postmenopausal genitourinary atrophy | Postmenopausal woman with decreased libido; most likely pelvic exam finding. | in review |
| 56 | Social Sciences / Communication | Exploring patient understanding | Long-standing type 1 diabetic with A1c 12% apologizes; best initial physician response. | in review |
| 57 | Biostatistics / Epidemiology | Sample size and power | Investigators triple enrollment in a finasteride prostate cancer trial; effect on error rates. | in review |
| 58 | Renal / Urinary | Ureteropelvic junction obstruction | Woman with recurrent UTIs, flank pain, hydronephrosis on MRI (image); site of obstruction. | in review |
| 59 | Immune / Infectious | Mendelian susceptibility to mycobacterial disease | Boy with disseminated Mycobacterium avium osteomyelitis and arthritis, HIV negative; expected immune defect. | in review |
| 60 | Musculoskeletal / Pathology | Recurrent fibromatosis (desmoid) | Woman with recurrent chest-wall subcutaneous mass at prior resection site; photomicrograph; cell of origin. | in review |

### Block 4

| # | System | Topic | What the item asks | Explanation |
|---|---|---|---|---|
| 61 | Reproductive / Pharmacology | ACE inhibitor fetopathy | Hypertensive woman on lisinopril with unrecognized pregnancy; fetus shows oligohydramnios and anomalies; mechanism. | in review |
| 62 | Cardiovascular / Respiratory | Acute pulmonary edema from MI | Woman dies of sudden respiratory distress; autopsy lung photomicrograph; underlying cause of death. | in review |
| 63 | General Principles / Pharmacology | Warfarin-rifampin interaction | Man on warfarin started on rifampin; INR stays 1 despite doubling warfarin dose; explanation. | in review |
| 64 | Nervous System / Special Senses | Age-related macular degeneration risk | Elderly woman with progressive central vision loss, funduscopic findings (chart format); strongest risk factor. | in review |
| 65 | Blood & Lymphoreticular / Immune | Chemotherapy-induced neutropenia | Child on ALL induction chemotherapy develops fever and gram-negative bacteremia; mechanism of susceptibility. | in review |
| 66 | Biostatistics / Epidemiology | Absolute risk reduction | RCT comparing wound infection rates of two closure procedures; compute absolute risk reduction. | in review |
| 67 | Blood & Lymphoreticular | Acute myeloid leukemia | Older smoker with fever, bleeding gums, petechiae, leukocytosis with monocytosis, thrombocytopenia; smear shown. | in review |
| 68 | Cardiovascular / Biochemistry | Familial hypercholesterolemia | Man with early MI, tendon xanthomas, cholesterol 410, strong family history; deficient protein. | in review |
| 69 | Renal / Urinary (Pediatrics) | Primary nocturnal enuresis | Five-year-old with nighttime wetting only, normal exam and urinalysis; underlying cause. | in review |
| 70 | Gastrointestinal (Pediatrics) | Rectal prolapse from constipation | Boy with hard stools, bright red blood, and protruding rectal mucosa; cause of finding. | in review |
| 71 | General Principles / Genetics | Carrier risk in autosomal recessive disease | Unaffected woman whose brother has cystic fibrosis; probability she is a carrier. | in review |
| 72 | Cardiovascular / Pharmacology | Vasopressor selection | Cardiogenic shock with tachycardia; drug to raise BP without raising heart rate; drug class. | in review |
| 73 | Renal / Urinary / Embryology | Horseshoe kidney | Girl with recurrent UTIs and a single U-shaped kidney; embryologic origin. | in review |
| 74 | Social Sciences / Communication | Adolescent sexual health interviewing | Teen worried a scrotal lesion is an STI; best initial physician response. | in review |
| 75 | Endocrine | Insulin resistance in obesity | Overweight woman with borderline fasting glucose and hypertension; serum abnormality versus normal-weight peer. | in review |
| 76 | Musculoskeletal / Renal | Hypokalemic periodic paralysis | Teen with episodic whole-body weakness after exercise and carbohydrate/salt loads; which electrolyte is abnormal. | in review |
| 77 | Endocrine / Renal | Central diabetes insipidus | Young man with polyuria, dilute urine that concentrates after vasopressin; dysfunctional structure. | in review |
| 78 | Reproductive / Oncology | Seminoma histology | Man with painless firm testicular mass; round cells with clear cytoplasm and central nuclei. | in review |
| 79 | General Principles / Embryology | Fetal alcohol syndrome | Newborn of mother with heavy nightly vodka use; expected examination finding. | in review |
| 80 | Endocrine | Postpartum thyroiditis | Woman 3 months postpartum with hyperthyroid symptoms, nontender goiter, no exophthalmos; mechanism. | in review |

### Block 5

| # | System | Topic | What the item asks | Explanation |
|---|---|---|---|---|
| 81 | Biostatistics / Epidemiology | False positives from specificity | Screening test with 25% sensitivity, 90% specificity applied to 100,000 women; compute false positives. | in review |
| 82 | Nervous System | Hemispatial neglect localization | Man bumping into left-sided objects, left lower quadrantanopia, left extinction; lesion lobe. | in review |
| 83 | Immune / Genetics | DiGeorge syndrome | Newborn with dysmorphic face, interrupted aortic arch, then hypocalcemic tetany; affected chromosome. | in review |
| 84 | Cardiovascular | Atrial arrhythmia origin (ECG) | Elderly malnourished man with palpitations and light-headedness; ECG shown; origin of abnormal rhythm. | in review |
| 85 | Gastrointestinal / Pharmacology | NSAID gastropathy | Woman on oral ibuprofen and topical diclofenac vomits blood with hypotension; underlying cause. | in review |
| 86 | General Principles / Embryology | Deformation vs malformation | Newborn with clubfoot after oligohydramnios from uteroplacental insufficiency; category of developmental disorder. | in review |
| 87 | Respiratory | Tension pneumothorax exam findings | Soldier with right chest stab wound and tracheal deviation; predict fremitus, percussion, breath sounds. | in review |
| 88 | Blood & Lymphoreticular | Reticulocytosis after hemorrhage | Young man loses 20% blood volume in trauma; expected smear finding at 10 days. | in review |
| 89 | Gastrointestinal / Nutrition | Fat-soluble vitamin deficiency | Man with steatorrhea, weight loss, and dry scaly rash; most likely deficient nutrient. | in review |
| 90 | Renal / Blood | Sickle cell papillary necrosis | Sickle cell woman with painless hematuria and papillary necrosis; medullary conditions raising her risk. | in review |
| 91 | Immune / Pathology | Acute inflammation mediators | Acute appendicitis specimen with neutrophilic infiltrate; mediator most responsible for microscopic findings. | in review |
| 92 | Behavioral Health | Brief psychotic disorder | Teen with sudden hallucinations and paranoia for one week after a stressor; negative tox screen. | in review |
| 93 | Nervous System / Infectious | Viral (aseptic) meningitis | Camper in summer with meningitis and lymphocytic CSF, normal glucose, mildly elevated protein. | in review |
| 94 | Respiratory | Neuromuscular restrictive lung disease | Cachectic elderly woman with reduced FEV1/FVC/TLC, elevated RV, normal DLCO, poor diaphragm excursion; cause. | in review |
| 95 | Reproductive / Pharmacology | Primary dysmenorrhea treatment | Young woman with severe first-day menstrual pain; mechanism of best initial drug. | in review |
| 96 | Renal / Urinary | Renal response to dehydration | Marathon runner collapses hot and volume-depleted; renal change that occurred before collapse. | in review |
| 97 | Multisystem / Infectious | Mumps | College freshman with fever and bilateral parotid swelling; causative agent. | in review |
| 98 | Immune / Oncology | Checkpoint inhibitor immunotherapy | Unresectable metastatic melanoma; antibody blockade of which molecule gives most benefit. | in review |
| 99 | Social Sciences / Ethics | Use of medical interpreters | Mandarin-only patient's husband insists on interpreting prognosis and consent; interpreter unavailable until morning. | in review |
| 100 | Respiratory / Infectious | Pertussis treatment | Unvaccinated infant with paroxysmal coughing spells, whoop, post-tussive emesis; drug class. | in review |

### Block 6

| # | System | Topic | What the item asks | Explanation |
|---|---|---|---|---|
| 101 | Nervous System / Pharmacology | Amphetamine mechanism | Amphetamine user with tremor, tachycardia, hypertension; source of sympathomimetic activity. | in review |
| 102 | Cardiovascular | Aortic dissection with aortic regurgitation | Tall teen collapses after severe back pain; hypotension with new diastolic murmur; cause of cardiac findings. | in review |
| 103 | Nervous System / Musculoskeletal | Lambert-Eaton myasthenic syndrome | Lung cancer patient with prolonged neuromuscular block; reduced EPP with normal MEPP; cause. | in review |
| 104 | Endocrine / Genetics | Familial hypocalciuric hypercalcemia | Asymptomatic man with familial hypercalcemia, elevated PTH, very low urinary calcium excretion; mutated gene. | in review |
| 105 | Social Sciences / Communication | Goals-of-care conversation | Woman with end-stage breast cancer refuses further thoracenteses; best physician response. | in review |
| 106 | Reproductive / Endocrine | 46,XX virilization | Newborn with ambiguous genitalia; if karyotype is 46,XX, most likely cause among listed options. | in review |
| 107 | Skin / Pharmacology | Tinea pedis therapy | Dishwasher with itchy, burning scaly feet (photo); enzyme inhibited by appropriate drug. | in review |
| 108 | Nervous System | Internuclear ophthalmoplegia | Young woman with diplopia; left eye fails to adduct on rightward gaze; damaged structure. | in review |
| 109 | Cardiovascular / Special Senses | Hypertensive retinopathy | Poorly controlled hypertensive man with headaches; expected ophthalmoscopic finding. | in review |
| 110 | Gastrointestinal (Pediatrics) | Neonatal cholestasis / biliary atresia | Two-week-old with jaundice, dark urine, predominantly direct hyperbilirubinemia, elevated GGT; cause. | in review |
| 111 | Renal / Cardiovascular | Renovascular hypertension in young women | 21-year-old with severe hypertension, abdominal bruit, LVH, normal labs; most likely cause. | in review |
| 112 | Biostatistics / Epidemiology | Normal distribution | Mean leukocyte count 7500 with SD 1000 in a normal distribution; range containing 50% of individuals. | in review |
| 113 | Blood & Lymphoreticular | Glanzmann thrombasthenia | Boy with mucosal bleeding, normal platelet count, normal ristocetin response, no aggregation to thrombin. | in review |
| 114 | General Principles / Genetics | Founder effect | Single BRCA2 mutation accounts for many Icelandic breast cancers; population-genetics explanation. | in review |
| 115 | Cardiovascular / Pathology | Cardiac myxoma | Woman with syncope episodes and a pedunculated intracardiac mass (photomicrograph); location. | in review |
| 116 | Immune / Transplant | Hyperacute rejection | Kidney transplant becomes swollen and hemorrhagic within minutes of reperfusion; cause. | in review |
| 117 | Musculoskeletal / Pharmacology | Bisphosphonate atypical femoral fracture | Myeloma patient on bisphosphonate has atraumatic midshaft femur fracture; underlying abnormality. | in review |
| 118 | Musculoskeletal | Lateral ankle sprain mechanism | Man twists ankle, tender just distal to lateral malleolus; mechanism of injury. | in review |
| 119 | Respiratory (Pediatrics) | Foreign body aspiration physiology | Toddler choking on candy with decreased left breath sounds and hypoxemia (x-ray); explanation for desaturation. | in review |

## Explanations

3 of 119 explanations are published; the rest are in review.

### Item 1 -- Inherited thrombophilia / pulmonary embolism

*Young woman post long flight with hypoxia, tachycardia, unilateral calf swelling; most likely underlying cause.*

## What the item is testing

The clinical picture is the easy half: immobility, sudden dyspnoea with hypoxia, sinus tachycardia and a swollen calf is a deep vein thrombosis that has embolised. The item does not stop there. It asks what made this particular patient clot when everyone else on that flight did not, which is a question about an inherited predisposition rather than about the trigger sitting in front of you.

## The reasoning

Activated protein C normally shuts the clotting cascade down by cleaving factor Va. A point mutation in the factor V gene removes one of the cleavage sites, so factor Va survives that brake and thrombin generation runs on. Long immobility supplies the venous stasis; the mutation supplies the missing off-switch. Together they produce exactly this vignette. Prevalence is what makes it the answer: heterozygous factor V Leiden is carried by a few percent of people of European ancestry, several times more often than any other inherited thrombophilia, so in a stem that asks for the *most likely* underlying cause it wins on base rate alone.

## Why the other directions fail

Antithrombin deficiency and protein C or S deficiency cause the same clinical event but are far rarer, and each usually arrives with its own tell -- apparent heparin resistance, or skin necrosis after starting warfarin. Platelet and von Willebrand factor defects push the patient the other way entirely, towards mucocutaneous bleeding, and cannot explain a clot.

## The pattern to keep

When a stem gives you a provoked venous clot in a young, otherwise healthy patient and asks for the underlying cause, answer with prevalence rather than with the most interesting disease. Factor V Leiden first; the prothrombin G20210A variant second.

**Sources**

- USMLE Step 1 sample test questions (booklet updated April 2026), item 1 and its answer key: https://www.usmle.org/exam-resources/step-1-materials/step-1-sample-test-questions
- Kujovich JL. Factor V Leiden thrombophilia. Genet Med. 2011;13(1):1-16.
- Bauer KA, Lip GYH. Overview of the causes of venous thrombosis. UpToDate.

### Item 2 -- Rheumatoid arthritis serology

*Middle-aged woman with years of hand stiffness and deformities (photo); which abnormality confirms diagnosis.*

## What the item is testing

A years-long symmetric small-joint arthritis with morning stiffness and established deformity of the fingers is rheumatoid arthritis clinically. The item is not asking you to make that diagnosis; it is asking which laboratory abnormality would confirm it. That is a question about test characteristics, and the distinction between a sensitive test and a specific one decides it.

## The reasoning

Citrullination is a normal post-translational change in which arginine residues are converted to citrulline. In rheumatoid arthritis, and largely only in rheumatoid arthritis, the immune system makes antibodies against those citrullinated peptides. That is why the anti-citrullinated peptide antibody is the serology that confirms: its specificity is high, so a positive result in a patient who already looks rheumatoid moves you close to certainty. It also appears early, often before joint destruction, and its presence marks patients whose disease is more likely to erode joints.

## Why the other directions fail

Rheumatoid factor is the reflex answer and the wrong one for a confirmation question. It is an antibody against the Fc portion of IgG, it is sensitive but not specific, and it turns up in Sjögren syndrome, chronic hepatitis C, endocarditis, other chronic infections and in healthy older adults. Antinuclear antibodies point towards lupus and are likewise too unspecific to confirm anything on their own. An elevated ESR or C-reactive protein tells you there is inflammation, which the swollen joints already told you.

## The pattern to keep

Read the verb in the question. "Screen" and "most sensitive" call for rheumatoid factor or an ANA; "confirm" and "most specific" call for the anti-citrullinated peptide antibody. The same pairing of a sensitive screen and a specific confirmation recurs across serology items.

**Sources**

- USMLE Step 1 sample test questions (booklet updated April 2026), item 2 and its answer key: https://www.usmle.org/exam-resources/step-1-materials/step-1-sample-test-questions
- Aletaha D, Neogi T, Silman AJ, et al. 2010 Rheumatoid arthritis classification criteria. Arthritis Rheum. 2010;62(9):2569-2581.
- Nishimura K, Sugiyama D, Kogata Y, et al. Meta-analysis: diagnostic accuracy of anti-cyclic citrullinated peptide antibody and rheumatoid factor for rheumatoid arthritis. Ann Intern Med. 2007;146(11):797-808.

### Item 3 -- Pontine stroke localization

*Elderly stroke patient with left abduction failure, right hemiparesis, right sensory loss; predict an additional finding.*

## What the item is testing

This is a localisation item wearing the clothes of a stroke item. You are given a crossed picture -- a cranial nerve deficit on one side, motor and sensory loss on the other -- and asked to predict a finding the vignette has not yet mentioned. Answering it means placing the lesion precisely enough that you can name what else sits at that level.

## The reasoning

Failure of abduction in one eye points to the abducens nerve or its nucleus, and the abducens nucleus lives in the caudal pons. Weakness and sensory loss on the opposite side of the body place the damage in the brainstem, above the pyramidal decussation at the cervicomedullary junction, where the descending corticospinal fibres have not yet crossed. The sensory findings are contralateral for the opposite reason: the medial lemniscus decussated as internal arcuate fibres back in the caudal medulla and the spinothalamic tract crossed in the cord, so both have already crossed by this level. Either way, the deficit lands on the side opposite the lesion. Both features are satisfied by a single caudal pontine lesion on the side of the eye finding, in the territory of paramedian branches of the basilar artery.

Once the lesion is at that level, the extra finding follows from local anatomy. Facial nerve fibres loop around the abducens nucleus as the internal genu before leaving the brainstem, so a lesion large enough to take out that nucleus generally takes the facial fibres with it. The result is a lower motor neuron facial palsy on the same side as the eye finding: the whole half of the face is weak, the eye will not close fully, and the smile is asymmetric.

## Why the other directions fail

A cortical or internal capsular lesion would produce contralateral face, arm and leg weakness together and would spare the forehead, with no isolated abduction failure. A midbrain lesion would trouble the oculomotor nerve, giving ptosis, a dilated pupil and an eye resting down and out.

## The pattern to keep

Crossed findings mean brainstem, and the cranial nerve involved gives the level: III means midbrain, VI and VII mean pons, IX to XII mean medulla. Then ask what else is packed into that few millimetres.

**Sources**

- USMLE Step 1 sample test questions (booklet updated April 2026), item 3 and its answer key: https://www.usmle.org/exam-resources/step-1-materials/step-1-sample-test-questions
- Blumenfeld H. Neuroanatomy Through Clinical Cases. 2nd ed. Sinauer; 2010. Brainstem vascular syndromes.
- Kim JS, Lee JH, Lee MC. Patterns of sensory dysfunction in lateral medullary and pontine infarction. Neurology. 1997;49(6):1557-1563.

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