Which of the following lung volumes cannot be determined by simple spirometry alone?
- A Tidal volume
- B Inspiratory reserve volume
- C Functional residual capacity ✓
- D Inspiratory capacity
Correct answer: C. Functional residual capacity
Explanation
Spirometry measures only volumes that exchange with the atmosphere: TV, IRV, ERV, IC, VC. Functional residual capacity contains the residual volume, the gas remaining after maximal expiration that never leaves the lungs, so FRC, RV and TLC all require helium dilution, nitrogen washout, or body plethysmography. Inspiratory capacity equals TV plus IRV, both directly measurable, which kills it as a distractor.
Reference: Ganong's Review of Medical Physiology, 26th ed.
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