You’re cruising through a heme/onc Q-bank and a stem screams: older patient, fatigue, infections, bruising, “macrocytosis,” and a weird smear. You pick the obvious answer… and then the explanation spends 80% of its time roasting every distractor. That’s not cruelty—that’s Step-style learning. Myelodysplastic syndromes (MDS) are a classic “cytopenias + dysplasia” diagnosis, and the wrong answers are usually other causes of cytopenias or marrow failure. Let’s break down a representative vignette and make every choice teach you something.
Tag: Heme/Onc > WBC Disorders & Lymphomas
The Vignette (Q-bank style)
A 72-year-old man presents with progressive fatigue and easy bruising for 4 months. He has had two “bad colds” this winter requiring antibiotics. He has no GI symptoms. Medications include only lisinopril. Exam shows pale conjunctiva and scattered petechiae.
Labs:
- Hemoglobin: 8.4 g/dL
- MCV: 108 fL
- Platelets: 62,000/µL
- WBC: 2,400/µL (ANC 700/µL)
- Reticulocyte count: low
Peripheral smear shows macro-ovalocytes, hypogranular neutrophils, and occasional bilobed neutrophils. Bone marrow biopsy is hypercellular with dysplastic erythroid and myeloid precursors.
Question: What is the most likely diagnosis?
Correct Answer: Myelodysplastic Syndromes (MDS)
Why MDS fits best
MDS is a clonal hematopoietic stem cell disorder characterized by:
- Ineffective hematopoiesis → cytopenias (often despite a hypercellular marrow)
- Morphologic dysplasia in one or more lineages
- Risk of progression to acute myeloid leukemia (AML)
This vignette is basically shouting MDS:
- Older age (most common in elderly)
- Pancytopenia (anemia + neutropenia + thrombocytopenia)
- Low reticulocytes (marrow isn’t producing effective mature cells)
- Hypercellular marrow (lots of precursors, but they’re dysfunctional)
- Dysgranulopoiesis:
- Hypogranular neutrophils
- Pseudo–Pelger-Huët anomaly (bilobed neutrophils)
High-yield MDS facts (USMLE-ready)
Clinical
- Symptoms reflect cytopenias:
- Anemia → fatigue, pallor
- Neutropenia → recurrent infections
- Thrombocytopenia → petechiae, bruising
- Can be primary (idiopathic) or secondary after:
- Chemotherapy (alkylators, topoisomerase II inhibitors)
- Radiation
- Benzene exposure (classically marrow-toxic)
Labs/Path
- Macrocytosis is common (often non-megaloblastic pattern, but can resemble megaloblastic indices)
- Hypercellular or sometimes normocellular marrow with ineffective hematopoiesis
- Increased blasts can occur; ≥20% blasts = AML
Genetics/associations
- Chromosomal abnormalities are common (Step tends to test that “this is clonal”)
- A famous one to know clinically: 5q deletion (5q- syndrome)
- Often macrocytic anemia, normal/high platelets in that subtype
- Lenalidomide can be effective (classically associated)
Key concept to say out loud in your head
MDS = dysplasia + cytopenias + (often) hypercellular marrow.
The marrow is “busy” but bad at making functional mature cells.
The Distractors: Why They’re Wrong (and how to spot them fast)
Below are common answer choices paired with what would need to be different in the vignette.
1) Aplastic anemia
Why students pick it: pancytopenia + infections + bruising.
Why it’s wrong here:
- Aplastic anemia is hypocellular marrow with fatty replacement.
- Peripheral smear does not feature dysplasia patterns like pseudo–Pelger-Huët.
What would make aplastic anemia correct
- History of drug/toxin exposure (chloramphenicol, benzene), radiation, viral hepatitis, EBV, HIV
- Very low reticulocytes (yes) plus hypocellular marrow (the key discriminator)
One-liner
- Aplastic anemia = empty marrow. MDS = crowded, dysplastic marrow.
2) Vitamin B12 deficiency (megaloblastic anemia)
Why students pick it: macrocytosis + anemia.
Why it’s wrong here:
- B12 deficiency can cause pancytopenia, but it classically shows:
- Hypersegmented neutrophils (not bilobed pseudo–Pelger-Huët)
- Megaloblastic changes without clonal dysplasia features
- You’d expect supportive history: neuropathy, malabsorption, pernicious anemia.
What would make B12 deficiency correct
- Neurologic symptoms (posterior column signs, peripheral neuropathy)
- Labs: low B12, ↑ methylmalonic acid and ↑ homocysteine
- Smear: hypersegmented neutrophils
Test-day trick
- Hypersegmented neutrophils = megaloblastic anemia
- Hyposegmented/bilobed (pseudo–Pelger-Huët) + hypogranular = MDS
3) Folate deficiency
Why students pick it: macrocytosis + cytopenias.
Why it’s wrong here:
- Like B12, folate deficiency causes hypersegmented neutrophils, not pseudo–Pelger-Huët.
- Folate deficiency lacks the neuro findings of B12 but otherwise overlaps.
What would make folate deficiency correct
- Risk factors: alcoholism, pregnancy, malnutrition, drugs (methotrexate, TMP-SMX, phenytoin)
- Labs: ↑ homocysteine, normal methylmalonic acid
4) Acute myeloid leukemia (AML)
Why students pick it: older age + cytopenias.
Why it’s wrong here:
- MDS can progress to AML, but AML requires ≥20% blasts in marrow (or blood), often with:
- Auer rods
- Prominent blasts on smear
- The vignette emphasizes dysplasia and ineffective hematopoiesis rather than a blast-driven picture.
What would make AML correct
- Markedly elevated blasts, constitutional symptoms, gum hypertrophy (monocytic variants), DIC in APL
- Severe marrow failure picture with blasts dominating
USMLE pearl
- MDS is a preleukemic clonal disorder.
- AML is when blasts take over.
5) Chronic lymphocytic leukemia (CLL)
Why students pick it: older patient + leukopenia? (and because CLL shows up everywhere)
Why it’s wrong here:
- CLL is a B-cell neoplasm that typically presents with:
- Lymphocytosis (not leukopenia)
- Lymphadenopathy, hepatosplenomegaly
- Smudge cells on smear
- Cytopenias in CLL are usually later (marrow infiltration or autoimmune hemolysis/thrombocytopenia).
What would make CLL correct
- Absolute lymphocytosis, painless LAD, smudge cells
- Flow: CD5+ CD23+ B cells
6) Hairy cell leukemia
Why students pick it: pancytopenia + infections.
Why it’s wrong here:
- Hairy cell leukemia classically gives:
- Massive splenomegaly
- “Dry tap” on marrow aspirate (fibrosis)
- “Hairy” cytoplasmic projections
- Often monocytopenia
- Not a dysplasia-first story.
What would make it correct
- Middle-aged man, splenomegaly, dry tap, TRAP+ (older classic), BRAF mutation (high yield conceptually)
7) Myelofibrosis
Why students pick it: marrow problems + cytopenias.
Why it’s wrong here:
- Myelofibrosis → fibrotic marrow → extramedullary hematopoiesis, typically:
- Teardrop cells (dacrocytes)
- Massive splenomegaly
- Often a “dry tap”
- This vignette gives hypercellular dysplastic marrow, not fibrotic replacement.
What would make it correct
- Splenomegaly + teardrop cells + leukoerythroblastic smear (nucleated RBCs, early myeloid forms)
8) Paroxysmal nocturnal hemoglobinuria (PNH)
Why students pick it: cytopenias and marrow failure associations.
Why it’s wrong here:
- PNH is driven by complement-mediated hemolysis due to lack of GPI-anchored proteins (CD55/CD59).
- You’d expect:
- Hemoglobinuria, thrombosis (hepatic vein/Budd-Chiari), smooth muscle dystonia symptoms
- Evidence of hemolysis: ↑ LDH, ↓ haptoglobin, ↑ indirect bilirubin
What would make it correct
- Dark urine in the morning, thromboses in unusual sites, flow cytometry showing ↓ CD55/CD59
Rapid Comparison Table (how Step wants you to sort these)
| Disorder | Marrow cellularity | Key smear clue | Typical “tell” |
|---|---|---|---|
| MDS | Hypercellular (often) | Pseudo–Pelger-Huët, hypogranular neutrophils | Elderly + cytopenias + dysplasia |
| Aplastic anemia | Hypocellular | No dysplasia | Toxins/drugs/viral; fatty marrow |
| B12/Folate deficiency | Hypercellular possible | Hypersegmented neutrophils | B12 neuro sx; folate risk factors |
| AML | Hypercellular | Blasts, ± Auer rods | ≥20% blasts |
| Myelofibrosis | Variable, often fibrotic | Teardrop cells | Massive splenomegaly, dry tap |
| CLL | Marrow infiltration later | Smudge cells | Lymphocytosis + LAD |
| Hairy cell leukemia | Often fibrotic | Hairy projections | Splenomegaly, dry tap |
| PNH | Variable | Not specific | Hemolysis + thrombosis; ↓ CD55/59 |
Exam-Day “If you see this, think MDS”
- Elderly patient + unexplained cytopenias
- Macrocytosis with low retic
- Smear with dysplastic neutrophils:
- Pseudo–Pelger-Huët (hyposegmented/bilobed)
- Hypogranular neutrophils
- Marrow: hypercellular with dysplasia (ineffective hematopoiesis)
- Mention of prior chemo/radiation (therapy-related MDS)
Takeaway
MDS is a clonal stem cell disorder where the marrow is often hypercellular but unproductive, producing dysplastic cells that die off or malfunction—leading to cytopenias and a risk of AML. The distractors mostly fail on one of three axes: cellularity (aplastic), smear morphology (hypersegmented vs pseudo–Pelger-Huët), or blast burden (AML).