USMLE: Hematology/Oncology – Myeloproliferative Disorders (PV, ET, Myelofibrosis, CML)

What This Is and Why It Matters for USMLE

Myeloproliferative disorders (MPDs) are a group of blood cancers characterized by excessive production of mature blood cells. They are high-yield for Step 1 and Step 2 CK, with a focus on classic presentation, diagnostic approach, and first-line treatment. MPDs are less commonly tested on Step 3, but appear in CCS scenarios.

High-Yield Facts (What You Must Memorize)

• Pathophysiology: MPDs result from mutations in genes involved in blood cell production, leading to uncontrolled proliferation.
• Classic presentation:
  • PV: Splenomegaly, anemia, thrombocytosis
  • ET: Splenomegaly, thrombocytosis, leukocytosis
  • Myelofibrosis: Splenomegaly, anemia, leukopenia
  • CML: Splenomegaly, leukocytosis, basophilia
• Diagnostic approach:
  • CBC with differential
  • Peripheral smear
  • Bone marrow biopsy
  • Genetic testing (JAK2, CALR, MPL mutations)
• First-line treatment:
  • PV: Phlebotomy, hydroxyurea
  • ET: Phlebotomy, anagrelide
  • Myelofibrosis: JAK inhibitors (ruxolitinib)
  • CML: Tyrosine kinase inhibitors (imatinib)
• Red flags, complications, and follow-up:
  • PV: Thrombosis, bleeding
  • ET: Thrombosis, bleeding
  • Myelofibrosis: Splenic rupture, anemia
  • CML: Blast crisis, resistance to therapy

Clinical Pearls & Buzzwords

• JAK2 mutation: Common in PV, ET, and myelofibrosis
• CALR mutation: Common in ET and myelofibrosis
• MPL mutation: Rare in MPDs
• Basophilia: Common in CML

Step-by-Step Clinical Reasoning

1. Identify the syndrome or presentation:
   • PV: Splenomegaly, anemia, thrombocytosis
   • ET: Splenomegaly, thrombocytosis, leukocytosis
   • Myelofibrosis: Splenomegaly, anemia, leukopenia
   • CML: Splenomegaly, leukocytosis, basophilia
2. Generate a differential (most likely and must-not-miss):
   • PV: Essential thrombocythemia, myelofibrosis
   • ET: PV, myelofibrosis
   • Myelofibrosis: PV, ET
   • CML: Acute lymphoblastic leukemia, acute myeloid leukemia
3. Order appropriate initial tests:
   • CBC with differential
   • Peripheral smear
   • Bone marrow biopsy
   • Genetic testing (JAK2, CALR, MPL mutations)
4. Interpret results:
   • JAK2 mutation: Common in PV, ET, and myelofibrosis
   • CALR mutation: Common in ET and myelofibrosis
   • MPL mutation: Rare in MPDs
5. Initiate treatment and monitoring:
   • PV: Phlebotomy, hydroxyurea
   • ET: Phlebotomy, anagrelide
   • Myelofibrosis: JAK inhibitors (ruxolitinib)
   • CML: Tyrosine kinase inhibitors (imatinib)

Common Mistakes & Exam Traps

• Mistake: Failing to consider JAK2 mutation in PV, ET, and myelofibrosis.
  • Why it happens: Rushing through the differential diagnosis.
  • How to avoid it: Take time to consider genetic testing results.
• Mistake: Failing to order bone marrow biopsy in suspected myelofibrosis.
  • Why it happens: Overreliance on peripheral smear.
  • How to avoid it: Consider the patient's presentation and lab results.
• Mistake: Failing to initiate phlebotomy in PV.
  • Why it happens: Focusing on hydroxyurea as first-line treatment.
  • How to avoid it: Consider the patient's hemoglobin level and symptoms.
• Mistake: Failing to consider blast crisis in CML.
  • Why it happens: Overreliance on tyrosine kinase inhibitors.
  • How to avoid it: Monitor for signs of disease progression.

How It’s Tested on USMLE

• Step 1: Basic science vignette (e.g., molecular mechanism, pathology slide, pharmacology).
  • Focus on pathophysiology, genetic testing, and treatment options.
• Step 2 CK: Clinical vignette (e.g., "A 45-year-old with chest pain...").
  • Focus on diagnosis, treatment, and monitoring.
• Step 3: Similar to Step 2 CK, plus prognosis, risk factors, and CCS management.
  • Focus on treatment decisions, monitoring, and patient outcomes.

CCS (Step 3) Relevance (If Applicable)

• Initial orders:
  • CBC with differential
  • Peripheral smear
  • Bone marrow biopsy
  • Genetic testing (JAK2, CALR, MPL mutations)
• Monitoring and follow-up:
  • Regular CBC with differential
  • Monitoring for signs of disease progression
  • Adjusting treatment as needed
• Common mistakes:
  • Failing to order indicated tests (e.g., bone marrow biopsy)
  • Delaying treatment (e.g., phlebotomy in PV)

Practice Questions (3-5 single-best-answer)

Question 1: A 55-year-old woman presents with splenomegaly, anemia, and thrombocytosis. Which of the following is the most likely diagnosis? A) Essential thrombocythemia B) Myelofibrosis C) Polycythemia vera D) Chronic myeloid leukemia

Answer: C) Polycythemia vera

Explanation: The patient's presentation is classic for PV, with splenomegaly, anemia, and thrombocytosis. The JAK2 mutation is common in PV, making it the most likely diagnosis.

Question 2: A 45-year-old man presents with splenomegaly, leukocytosis, and basophilia. Which of the following is the most likely diagnosis? A) Acute lymphoblastic leukemia B) Acute myeloid leukemia C) Chronic myeloid leukemia D) Myelofibrosis

Answer: C) Chronic myeloid leukemia

Explanation: The patient's presentation is classic for CML, with splenomegaly, leukocytosis, and basophilia. The JAK2 mutation is rare in CML, making it less likely.

Question 3: A 60-year-old woman presents with splenomegaly, anemia, and leukopenia. Which of the following is the most likely diagnosis? A) Essential thrombocythemia B) Myelofibrosis C) Polycythemia vera D) Chronic myeloid leukemia

Answer: B) Myelofibrosis

Explanation: The patient's presentation is classic for myelofibrosis, with splenomegaly, anemia, and leukopenia. The JAK2 mutation is common in myelofibrosis, making it the most likely diagnosis.

Quick Reference Card (60-Second Summary)

• PV: Phlebotomy, hydroxyurea
• ET: Phlebotomy, anagrelide
• Myelofibrosis: JAK inhibitors (ruxolitinib)
• CML: Tyrosine kinase inhibitors (imatinib)
• JAK2 mutation: Common in PV, ET, and myelofibrosis
• CALR mutation: Common in ET and myelofibrosis

If You Get Stuck on Test Day

• Eliminate obviously wrong answers (e.g., AML in a patient with basophilia).
• Use the "next best step" hierarchy (least invasive, most specific).
• For Step 3 CCS: order basic labs, vitals, and IV access when unsure.

Related USMLE Topics

• Leukemia: Connects to MPDs through shared pathophysiology and treatment options.
• Thrombocytopenia: Connects to MPDs through shared risk factors and treatment options.
• Splenomegaly: Connects to MPDs through shared presentation and diagnostic approach.