✅ Correct Answer: C) ATP synthase The cell shows normal glycolysis, normal TCA cycle, and normal oxygen consumption. This tells you the ETC complexes (I–IV) are functioning and pumping protons normally. However, ATP production is almost zero, meaning the cell cannot use the proton gradient to make ATP. That step is performed by ATP synthase (Complex V) → the most likely defect. The clue: - Adding an artificial proton carrier restores ATP synthesis → confirms that the ETC is intact and the issue is specifically with ATP generation, not proton pumping. Why the other choices are wrong - A) Pyruvate dehydrogenase → would decrease TCA activity and O₂ consumption (not seen here). - B) Complex II → would affect ETC flow and O₂ use (but O₂ is normal). - D) Hexokinase → affects glycolysis (but glycolysis is normal). 💡 MCAT Takeaway: Normal O₂ use + almost no ATP = ATP synthase defect, not an ETC complex problem.