An athlete's complete blood count shows: Hb 17 g/dL, Hct 50%, RBC 5.8 × 10⁶/µL. Mean corpuscular volume (MCV) calculation yields 86 fL. Which of the following correctly interprets this blood picture?
- A Physiologic erythrocytosis from high-altitude training with normal cell size; secondary polycythemia due to EPO-driven erythroid expansion ✓
- B Polycythemia vera with macrocytosis suggesting B12 deficiency as a contributing factor
- C Spurious polycythemia due to dehydration; MCV of 86 fL indicates microcytosis
- D Iron deficiency anemia compensated by increased RBC count
Correct answer: A. Physiologic erythrocytosis from high-altitude training with normal cell size; secondary polycythemia due to EPO-driven erythroid expansion
Explanation
MCV = (Hct/RBC) × 10 = (50/5.8) × 10 = 86.2 fL, which is normocytic (normal 80–100 fL). Elevated Hb, Hct, and RBC in an athlete at altitude reflect appropriate EPO-stimulated erythropoiesis from chronic hypoxic stimulus, categorized as physiologic secondary polycythemia. Polycythemia vera involves JAK2 mutation with EPO-independent marrow activation and is typically associated with thrombocytosis and leukocytosis, not seen here.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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