Respiratory System MCAT Practice Question
A 34-year-old male with no significant medical history undergoes cardiopulmonary exercise testing. During peak exercise, his arterial blood gas shows pH 7.32, pCO₂ 48 mmHg, HCO₃⁻ 24 mEq/L, and lactate 8 mmol/L. His minute ventilation increases appropriately to 120 L/min. The examining physician notes that patients with carbonic anhydrase deficiency develop severe metabolic complications during exercise despite normal lung function. Carbonic anhydrase catalyzes the reversible reaction: CO₂ + H₂O ↔ H₂CO₃ ↔ H⁺ + HCO₃⁻ with a turnover number (kcat) of approximately 10⁶ s⁻¹, the fastest of any known enzyme. Which of the following best explains why this extraordinarily high catalytic rate is physiologically essential?
Answer choices
- AIt prevents spontaneous CO₂ hydration in plasma, which would cause immediate precipitation of carbonic acid crystals in capillaries
- BIt enables rapid conversion of metabolic CO₂ to HCO₃⁻ in RBCs, allowing efficient CO₂ transport from tissues to lungs within the transit time across tissue capillariesCorrect answer
- CIt maintains intracellular pH above 7.4 by buffering all H⁺ ions produced from aerobic metabolism before they can affect hemoglobin oxygen affinity
- DIt directly enhances hemoglobin's oxygen-binding capacity by catalyzing conformational changes in the globin chains during exercise
- EIt creates a rate-limiting step that prevents hyperventilation and respiratory alkalosis during intense physical activity
- FIt regenerates CO₂ from HCO₃⁻ in pulmonary capillaries, allowing exhalation of gaseous CO₂ at a rate matched to metabolic production
See the full explanation
Get the correct-answer rationale, why each distractor is wrong, the underlying mechanism, and high-yield associations — plus adaptive practice that targets your weak areas — with a free MedBoardPRO account.