Amobarbital (a barbiturate) and rotenone both inhibit the same complex of the electron transport chain. Which complex and site of inhibition do they share?

• A Complex II, blocking electron transfer from FADH2 to ubiquinone
• B Complex III, blocking the Q cycle
• C Complex IV, blocking electron transfer to molecular oxygen
• D Complex I, blocking electron transfer from NADH to ubiquinone ✓

Explanation

Both amobarbital and rotenone inhibit Complex I (NADH:ubiquinone oxidoreductase), blocking electron transfer from NADH to ubiquinone. This is distinct from antimycin A (Complex III) and cyanide (Complex IV). Distractor A is wrong because Complex II is not inhibited by these agents. Distractor C describes cyanide or carbon monoxide poisoning.

Reference: Katzung Basic and Clinical Pharmacology, 15th ed.

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