Question

A 5-year-old girl was washing her doll with shampoo containing a chemical Rotenone. Her mother notices her in an unconscious state. Which of the following complex is inhibited by the above-mentioned chemical?

• A. NADH Dehydrogenase
• B. Succinate DH
• C. Cytochrome C
• D. Cytochrome oxidase

Hint:

Rotenone inhibits NADH dehydrogenase, disrupting ATP production and leading to cellular dysfunction and toxicity.

Answer 1

The Correct Option is A

Step 1: Spot the toxicology trigger.
Rotenone is a classic mitochondrial poison used in pesticides and some shampoos. Sudden unconsciousness after exposure points to a sudden collapse of cellular energy (ATP) production, so the lesion must sit on the electron transport chain (ETC).

Step 2: Recall where Rotenone acts mechanistically.
Rotenone is a lipophilic molecule that plugs the ubiquinone (coenzyme Q) binding pocket of the very first ETC pump. It therefore blocks the handoff of electrons from NADH-fed iron-sulfur clusters to coenzyme Q. With this gate shut, the entire NADH-linked arm of respiration stalls.

Step 3: Match the block to a numbered complex.
The pump that accepts electrons from NADH and passes them to coenzyme Q is Complex I, also called NADH dehydrogenase (NADH:ubiquinone oxidoreductase). This is exactly the protein Rotenone disables.

Step 4: Confirm the other complexes are bystanders.
$\bullet$ Succinate dehydrogenase (Complex II) feeds electrons via FADH$_2$ and bypasses the Rotenone block, so it keeps working.
$\bullet$ Cytochrome c is only a mobile shuttle between Complexes III and IV, not a Rotenone target.
$\bullet$ Cytochrome oxidase (Complex IV) is the cyanide/CO target, not the Rotenone target.

Step 5: Conclusion.
Because Rotenone selectively jams the NADH $\rightarrow$ coenzyme Q step at the chain's entry pump, the inhibited complex is NADH dehydrogenase (Complex I).

Final answer: Option 1 - NADH Dehydrogenase.