FADH2 enters at which complex in ETC ?

**Core Concept**
The electron transport chain (ETC) is a series of protein complexes located in the mitochondrial inner membrane, responsible for generating ATP during oxidative phosphorylation. FADH2 is a reduced electron carrier that donates electrons to the ETC.

**Why the Correct Answer is Right**
FADH2 enters the ETC at complex II (succinate dehydrogenase), also known as the succinate:quinone oxidoreductase. This complex is unique in that it is both a part of the ETC and the citric acid cycle (Krebs cycle). FADH2 is formed during the conversion of succinate to fumarate in the citric acid cycle. The electrons from FADH2 are then passed to ubiquinone (CoQ), which is the electron carrier that links complex I and complex III in the ETC.

**Why Each Wrong Option is Incorrect**
* **Option A:** This is incorrect because complex I is where NADH enters the ETC, not FADH2.
* **Option B:** This is incorrect because complex III is where the electrons from ubiquinol (reduced CoQ) are passed to cytochrome c, not FADH2.
* **Option D:** This is incorrect because complex IV is where the electrons from cytochrome c are passed to oxygen, the final electron acceptor in the ETC.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that FADH2 enters the ETC at complex II, which is the same complex involved in the citric acid cycle. This highlights the intimate relationship between the ETC and the citric acid cycle, two crucial processes in cellular respiration.

**Correct Answer: C. Complex II.