An increase in the blood's oxygen-carrying capacity provides the most important longer term adjustment to altitude exposure. Which factors accounts for this adaptation?
A. Initial decrease in plasma volume.
B. Initial increase in plasma volume.
C. Increase in erythrocytes.
D. Increase in hemoglobin synthesis.

Correct option is D

The correct answer is (d).
Introduction
Long‑term altitude adaptation enhances oxygen‑carrying capacity via haemoconcentration (initial plasma volume decrease), increased erythrocyte production, and elevated haemoglobin synthesis. These collectively raise arterial oxygen content. Initial plasma volume increase (B) dilutes blood and is not part of this adaptation.
Information Booster

• Initial decrease in plasma volume (A) raises haematocrit.
• Increase in erythrocytes (C) boosts red blood cell count.
• Increase in haemoglobin synthesis (D) enhances oxygen‑binding capacity.
• Plasma volume increase (B) would lower oxygen‑carrying capacity.

• Option A includes B (incorrect).
• Option B includes B and excludes A.
• Option C excludes D, which is essential.