A 58-year-old man has lived and worked as a miner at 4300 m in the Andes for 25 years. He presents with severe headache, dizziness, marked cyanosis, and exertional dyspnoea. Haemoglobin is 23.5 g/dL, haematocrit 72 percent, SaO2 78 percent, and PaCO2 is elevated at 48 mmHg. Which of the following best explains his condition?

• A Pulmonary hypertension with right-to-left shunting through a patent foramen ovale
• B Sustained activation of carotid body chemoreceptors causing extreme hyperventilation and respiratory alkalosis
• C Loss of the hypoxic ventilatory response with alveolar hypoventilation causing exaggerated hypoxaemia ✓
• D Renal retention of bicarbonate causing metabolic alkalosis that suppresses ventilation

Explanation

Chronic mountain sickness (Monge disease) occurs in long-term high-altitude residents when blunted carotid body sensitivity leads to hypoventilation, a raised PaCO2, more severe hypoxaemia than in acclimatised peers, and excessive erythrocytosis. Treatment is descent, phlebotomy, or acetazolamide. Option B is wrong because the defining abnormality is reduced, not increased, ventilatory drive, and the patient is hypocapnic in pure acclimatisation. Option A describes an acute structural event, not a chronic maladaptation.

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

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