Polycythemia vera (PV) is one of those Step-style diagnoses that seems straightforward—until the question writer starts sprinkling in pruritus, splenomegaly, “high EPO vs low EPO,” thrombosis risk, and a tempting list of myeloproliferative look-alikes. The real skill isn’t just picking the correct answer—it’s knowing why every other answer is wrong.
The Vignette (Q-bank style)
A 62-year-old man presents with headaches, dizziness, and generalized itching that worsens after hot showers. He has facial plethora and mild splenomegaly on exam. Labs show:
- Hemoglobin: 19.2 g/dL (elevated)
- Hematocrit: 58% (elevated)
- WBC: 14,000/µL (mildly elevated)
- Platelets: 650,000/µL (elevated)
- Serum erythropoietin (EPO): low
- Peripheral smear: increased RBC mass, no significant schistocytes
- Oxygen saturation: normal
Question: What is the most likely underlying mechanism?
Answer choices
A. JAK2 tyrosine kinase mutation causing constitutive hematopoiesis
B. Increased EPO production due to chronic hypoxemia
C. BCR-ABL fusion leading to increased granulocyte proliferation
D. Increased RBC mass due to relative hemoconcentration from dehydration
E. EPO secretion by renal cell carcinoma
Correct Answer: A. JAK2 tyrosine kinase mutation causing constitutive hematopoiesis
PV is a myeloproliferative neoplasm (MPN) driven most commonly by a JAK2 mutation (classically JAK2 V617F), leading to EPO-independent red cell production.
Why PV fits this vignette
Classic PV clues:
- Aquagenic pruritus (itching after hot showers): basophil/mast cell mediator release
- Plethora + headaches/dizziness: hyperviscosity symptoms
- Splenomegaly: extramedullary hematopoiesis
- Low EPO: negative feedback because RBC mass is high
- Increased WBC and platelets: panmyelosis (not purely RBCs)
Core pathophysiology (high-yield)
- JAK2 activation → hypersensitivity to growth factors (EPO signaling pathway is “stuck on”)
- Bone marrow becomes hypercellular with trilineage growth (RBCs, WBCs, platelets)
Big complications to remember
- Thrombosis (e.g., DVT, stroke, Budd-Chiari syndrome)
- Bleeding can also occur (platelets can be numerous but dysfunctional)
- Progression risks: myelofibrosis (“spent phase”) or acute leukemia
Treatment anchors for Step
- Phlebotomy to reduce hematocrit (goal often <45%)
- Hydroxyurea (cytoreduction in high-risk patients)
- Low-dose aspirin (thrombosis prevention; careful in bleeding risk)
- Ruxolitinib (JAK inhibitor) in select cases
Why Each Distractor Is Wrong (and what it’s testing)
B. Increased EPO production due to chronic hypoxemia
This describes secondary polycythemia from low oxygen, not PV.
How to spot it:
- Chronic lung disease, OSA, cyanotic heart disease, high altitude
- Low O saturation (often)
- High EPO (key differentiator)
In this vignette: oxygen saturation is normal and EPO is low, which argues strongly against hypoxemia-driven erythrocytosis.
Step tip: If EPO is high, think “secondary.” If EPO is low, think “primary” (PV).
C. BCR-ABL fusion leading to increased granulocyte proliferation
This is describing chronic myeloid leukemia (CML).
CML hallmarks:
- BCR-ABL t(9;22) Philadelphia chromosome
- Marked leukocytosis with left shift, basophilia
- Often low leukocyte alkaline phosphatase (LAP)
- Symptoms: fatigue, weight loss, night sweats, splenomegaly
Why it’s wrong here:
- The primary abnormality is RBC mass with aquagenic pruritus and low EPO—not an isolated granulocytic proliferation.
- PV can have mild leukocytosis, but RBC elevation + low EPO is the giveaway.
High-yield contrast table:
| Feature | Polycythemia vera | CML |
|---|---|---|
| Driver mutation | JAK2 (V617F common) | BCR-ABL |
| Main cell line | RBCs (often with ↑ WBC/plt too) | Granulocytes |
| EPO level | Low | Normal/variable |
| Classic clue | Aquagenic pruritus, plethora | Basophilia, very high WBC |
| First-line targeted drug | Ruxolitinib (selected) | Imatinib (TKI) |
D. Increased RBC mass due to relative hemoconcentration from dehydration
This is relative polycythemia, meaning the RBC mass is normal, but plasma volume is decreased.
Clues for relative polycythemia:
- Dehydration, diuretics, burns
- Hemoconcentration: elevated Hct/Hgb but no true increase in RBC mass
- No splenomegaly, no thrombocytosis/leukocytosis from panmyelosis
- EPO is not characteristically suppressed the way it is in PV
Why it’s wrong here:
- The vignette suggests a true myeloproliferative process: splenomegaly + thrombocytosis + leukocytosis + pruritus.
Step tip: Relative polycythemia = “looks like polycythemia on CBC,” but lacks PV’s systemic features and marrow panmyelosis.
E. EPO secretion by renal cell carcinoma
This is another form of secondary polycythemia, but driven by ectopic EPO production.
EPO-producing tumors (classic Step list):
- Renal cell carcinoma
- Hepatocellular carcinoma
- Cerebellar hemangioblastoma
- Uterine leiomyoma (less commonly tested)
How it would look:
- High EPO
- Possible tumor symptoms (hematuria, flank pain, weight loss) depending on the tumor
- No trilineage proliferation required
Why it’s wrong here: EPO is low.
High-Yield PV Facts You Should Be Ready to Use on Test Day
1) Primary vs Secondary erythrocytosis (the EPO split)
- Low EPO → primary (PV, or rare EPO receptor mutations)
- High EPO → secondary (hypoxia, tumors, exogenous EPO)
2) Pruritus after hot shower is a PV buzzword
- Think mast cell/basophil mediator release in an MPN context.
3) PV is an MPN that can look “panmyelotic”
- You don’t need isolated RBC elevation. WBC and platelets can be high, too.
4) Thrombosis is a major cause of morbidity/mortality
- Classic association: Budd-Chiari syndrome (hepatic vein thrombosis)
5) Treatment patterns are testable
- Phlebotomy + aspirin (common baseline strategy)
- Hydroxyurea for high-risk patients (older age, thrombosis history)
- JAK inhibitors may appear as “next step” in resistant disease
Quick Wrap-Up: How to nail PV questions fast
If you see aquagenic pruritus + splenomegaly + high Hct and the stem gives you low EPO, you’re living in polycythemia vera territory—a JAK2-driven myeloproliferative neoplasm. Then use the distractors to prove you understand the map:
- Hypoxia/tumors → high EPO
- Dehydration → relative, no MPN features
- CML → BCR-ABL, granulocytes dominate, different “buzzwords”