Normal A-a Gradient — FRCA Primary MCQ

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ModeratePhysiology - RespiratoryNormal A-a GradientFRCA Primary

A healthy 30-year-old is breathing room air at sea level. The expected alveolar–arterial oxygen gradient can be estimated as (age + 10)/4 mmHg. Given that 1 mmHg is approximately 0.133 kPa, which value is closest to the expected gradient?

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Correct answer: A — 1.3 kPa

Explanation lettering: E = shown as A · D = shown as B · B = shown as C · A = shown as D · C = shown as E

The correct answer is E. For a 30-year-old, the expected A–a oxygen gradient is (30 + 10)/4 = 10 mmHg. Multiplying by 0.133 converts this to approximately 1.3 kPa. A small physiological gradient exists because of normal ventilation–perfusion inequality and minor venous admixture; it is not normally zero, so A is too low. The expected gradient increases with age. Values of 3.3–9.3 kPa represented by C, B and D are substantially elevated for this healthy young adult and would suggest impaired pulmonary gas exchange, such as ventilation–perfusion mismatch, diffusion limitation or shunt. Pure alveolar hypoventilation lowers both alveolar and arterial oxygen tensions without substantially increasing the A–a gradient.

Reference: Kondamudi NP, Gopalakrishna A, Gopalakrishnan R. Physiology, Alveolar to Arterial Oxygen Gradient. StatPearls, 2026 edition. https://pubmed.ncbi.nlm.nih.gov/31424737/