You’re cruising through a heme question bank, you see “acute anemia after meds + dark urine,” and your brain screams G6PD deficiency. Great—but the real score jump comes when you can also explain why each distractor is wrong. That’s where test writers live.
Tag: Heme/Onc > RBC Disorders & Anemias
The Clinical Vignette (Classic Q-Bank Style)
A 24-year-old man develops fatigue, back pain, and dark urine 2 days after starting trimethoprim-sulfamethoxazole for a skin infection. He is African American. Exam shows scleral icterus. Labs show decreased hemoglobin, elevated LDH, low haptoglobin, and increased indirect bilirubin. Peripheral smear shows bite cells.
Question: What is the most likely underlying mechanism?
The Correct Answer: G6PD Deficiency (Oxidative Hemolysis)
What’s happening physiologically?
Glucose-6-phosphate dehydrogenase (G6PD) is the rate-limiting enzyme of the pentose phosphate pathway (PPP / HMP shunt), generating NADPH.
- NADPH keeps glutathione reduced
- Reduced glutathione detoxifies reactive oxygen species (ROS)
- Without enough NADPH → oxidant stress damages hemoglobin and RBC membranes → hemolysis
High-yield equation concept:
- PPP produces NADPH, which is essential for maintaining glutathione in its reduced form:
Why the vignette screams G6PD
Trigger + timing + hemolysis labs + smear
- Trigger: sulfonamides (also dapsone, primaquine, nitrofurantoin; fava beans; infections)
- Timing: hemolysis often occurs 1–3 days after oxidant stress
- Hemolysis pattern: typically intravascular + extravascular
- Smear:
- Heinz bodies (denatured oxidized Hb; seen with supravital stain)
- Bite cells (splenic macrophages “bite out” Heinz body–laden portions)
Inheritance + population
- X-linked recessive
- Classically affects individuals of African, Mediterranean, Middle Eastern ancestry
- The African (A−) variant often has episodic hemolysis with stressors rather than chronic baseline hemolysis
Lab pattern (know this cold)
| Finding | Direction | Why |
|---|---|---|
| LDH | ↑ | RBC destruction releases LDH |
| Indirect bilirubin | ↑ | heme breakdown |
| Haptoglobin | ↓ | binds free Hb in plasma |
| Reticulocytes | ↑ | marrow response |
| Coombs test | Negative | not immune-mediated |
Step tip: In acute hemolysis, G6PD level can be falsely normal because older, enzyme-deficient RBCs are destroyed first, leaving younger reticulocytes with higher G6PD activity.
Why Every Distractor Is Wrong (and How They Try to Trick You)
Below are common “near-miss” options that show up in the same question family.
Distractor 1: Hereditary Spherocytosis (Spectrin/Ankyrin Defect)
Why it’s tempting: Hemolysis + jaundice can happen.
Why it’s wrong here: The smear and trigger don’t match.
Key distinguishing features
- Autosomal dominant (most common)
- Membrane/cytoskeleton defect (spectrin, ankyrin, band 3, protein 4.2)
- Extravascular hemolysis (splenic sequestration)
- Smear: spherocytes (no central pallor), not bite cells
- Tests: ↑ MCHC, positive osmotic fragility or ↓ EMA binding test
- Classically associated with splenomegaly and pigment gallstones
Board clue: Spherocytosis = “sphere-shaped RBCs that can’t squeeze through spleen.”
Distractor 2: Autoimmune Hemolytic Anemia (Warm vs Cold) — Positive Coombs
Why it’s tempting: Hemolysis labs overlap (↑ LDH, ↓ haptoglobin, ↑ indirect bili).
Why it’s wrong here: This vignette points to oxidative damage, not antibodies.
Warm AIHA
- IgG-mediated, extravascular
- Associated with: SLE, CLL, penicillins/cephalosporins, methyldopa
- Smear: spherocytes
- Direct antiglobulin (Coombs) test: positive
Cold agglutinin disease
- IgM-mediated, complement
- Associated with: Mycoplasma pneumoniae, EBV, CLL
- Findings: acrocyanosis, RBC agglutination
- Coombs: positive for C3
Board clue: If they mention Coombs positive, you’re in immune territory—not G6PD.
Distractor 3: Sickle Cell Disease/Trait (HbS Polymerization)
Why it’s tempting: African ancestry + hemolysis.
Why it’s wrong here: The trigger and smear are off.
Sickle cell
- Point mutation in beta-globin: Glu → Val
- Polymerization with deoxygenation
- Smear: sickled cells, target cells; Howell-Jolly bodies if functional asplenia
- Triggers: hypoxia, dehydration, acidosis—not “new sulfa drug” as the main clue
- More chronic hemolysis, vaso-occlusive pain crises, acute chest syndrome, splenic infarction
Board clue: Dark urine after oxidant drug + bite cells = oxidative hemolysis, not sickling.
Distractor 4: Pyruvate Kinase Deficiency (↓ ATP → Rigid RBCs)
Why it’s tempting: Enzyme defect → hemolytic anemia.
Why it’s wrong here: Different pathway, different smear clues, usually more chronic.
Pyruvate kinase deficiency
- Autosomal recessive
- RBCs rely on glycolysis → ↓ ATP → membrane pumps fail → rigid RBCs → extravascular hemolysis
- Often presents in infancy/childhood with chronic hemolysis
- Smear may show echinocytes (burr cells)
- Lab nuance: ↑ 2,3-BPG → right shift → improved oxygen unloading (patients may tolerate anemia better than expected)
Board clue: PK deficiency is a glycolysis problem (ATP), not an oxidative stress problem (NADPH).
Distractor 5: Thalassemias (Decreased Globin Chain Synthesis)
Why it’s tempting: Common RBC disorder; anemia question banks love it.
Why it’s wrong here: The anemia type and smear pattern are different.
Thalassemia
- Microcytic anemia (low MCV)
- Smear: target cells, basophilic stippling (sometimes)
- No acute hemolysis after oxidant drugs is the typical story
- Beta-thal major: severe transfusion-dependent anemia, marrow expansion
- Alpha-thal: HbH (β4) or hydrops fetalis (γ4)
Board clue: Microcytosis + target cells ≠ bite cells after TMP-SMX.
Distractor 6: Paroxysmal Nocturnal Hemoglobinuria (PIGA Mutation)
Why it’s tempting: Dark urine and hemolysis.
Why it’s wrong here: The mechanism and clinical constellation are different.
PNH
- Acquired mutation in PIGA → absent GPI anchor
- Loss of complement inhibitors CD55 (DAF) and CD59
- Intravascular hemolysis → hemoglobinuria (often morning), thrombosis, aplastic anemia association
- Diagnosis: flow cytometry showing ↓ CD55/CD59 on RBCs/WBCs
- Not classically triggered immediately by sulfa drugs; smear doesn’t feature bite cells/Heinz bodies
Board clue: PNH = complement-mediated lysis + thrombosis risk.
High-Yield “If You See This, Think G6PD” Checklist
Triggers (memorize)
- Drugs: sulfonamides (TMP-SMX), dapsone, primaquine, nitrofurantoin
- Foods: fava beans
- Infections: oxidative burst from neutrophils ramps up ROS
Smear associations
- Heinz bodies: oxidized denatured Hb (special stain)
- Bite cells: splenic removal of Heinz bodies
Exam favorite pitfalls
- Coombs test is negative (not immune)
- G6PD assay may be normal during an acute episode
- Patients can be asymptomatic between episodes (especially African variant)
Quick Compare Table: G6PD vs Common Look-Alikes
| Disorder | Triggered episodes? | Smear clue | Coombs | Key mechanism |
|---|---|---|---|---|
| G6PD deficiency | Yes (drugs, fava, infection) | Bite cells, Heinz bodies | Negative | ↓ NADPH → oxidative damage |
| Hereditary spherocytosis | Often chronic | Spherocytes | Negative | Membrane defect → splenic trapping |
| Warm AIHA | Can be drug-related | Spherocytes | Positive | IgG-mediated |
| Sickle cell | Stress triggers (hypoxia) | Sickled cells | Negative | HbS polymerization |
| PK deficiency | Chronic | Burr cells | Negative | ↓ ATP → rigid RBCs |
| PNH | Chronic/episodic | Nonspecific | Negative | ↓ CD55/59 → complement lysis |
Takeaway (How to Win the Question)
When a vignette combines oxidant stress (sulfa/primaquine/dapsone, fava beans, infection) with hemolysis labs and bite cells, the test writer is telling you: PPP failure → ↓ NADPH → ↓ reduced glutathione → hemolysis.
And when you can confidently say why it’s not spherocytosis, AIHA, sickle, PK deficiency, thalassemia, or PNH—you’re no longer pattern-matching. You’re thinking like the exam.