During a single bout of high-altitude exposure (4500 m), erythropoietin (EPO) secretion increases within hours. What is the expected reticulocyte response after 5 days at altitude?

• A Reticulocyte count is markedly elevated, reflecting new red cell release from marrow
• B Reticulocyte count falls because hypoxia suppresses bone marrow release
• C Reticulocyte count remains at baseline because EPO has not yet stimulated erythropoiesis
• D Reticulocyte count is beginning to rise but is not yet markedly elevated ✓

Explanation

EPO rises within 2 to 3 hours of hypoxic exposure, peaks around 24 to 48 hours, and stimulates erythroid progenitors. However, maturation and release of reticulocytes from marrow takes about 3 to 5 days. By day 5, reticulocytes are just beginning to rise; marked elevation is seen after 7 to 10 days. The baseline option ignores the kinetics of erythroid maturation.

Reference: Ganong's Review of Medical Physiology, 26th ed.

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