A 32 year old high altitude rock climber is observed to have a hematocrit of 70 percent. Which of the following represents the MOST likely explanation?
**Core Concept**
Polycythemia is a condition characterized by an excessive increase in red blood cell mass, often resulting from chronic hypoxia at high altitudes. This condition is a compensatory response to low oxygen levels, aiming to enhance oxygen delivery to tissues.
**Why the Correct Answer is Right**
At high altitudes, the reduced oxygen availability triggers the release of erythropoietin (EPO) from the kidneys. EPO stimulates the bone marrow to produce more red blood cells, leading to an increase in hematocrit levels. In this case, the rock climber's hematocrit of 70 percent indicates a significant polycythemic response to chronic hypoxia.
**Why Each Wrong Option is Incorrect**
**Option A:** Dehydration would cause a decrease in plasma volume, which might temporarily increase hematocrit levels. However, dehydration would not explain the significant increase in red blood cell mass observed in this case.
**Option B:** Hyperthyroidism can cause an increase in red blood cell production, but it would not be the primary explanation for polycythemia in a high-altitude rock climber.
**Option C:** Chronic lung disease would indeed lead to hypoxia, but it would typically cause a decrease in oxygen levels in arterial blood, rather than an increase in red blood cell mass.
**Option D:** This option is incomplete.
**Clinical Pearl / High-Yield Fact**
At high altitudes, acclimatization involves a complex interplay of physiological adaptations, including increased red blood cell production, increased capillarization, and increased ventilation. A hematocrit level above 60 percent in someone living at high altitude is considered polycythemic.
**Correct Answer:** C.