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, crucial for generating ATP during oxidative phosphorylation. FADH2 is a high-energy electron carrier that plays a key role in the ETC. The correct location of FADH2 entry into the ETC is essential for understanding the process of energy production in cells.

**Why the Correct Answer is Right**
FADH2 enters the ETC at Complex II (also known as Succinate Dehydrogenase or Succinate:Quinone Oxidoreductase), which is a part of both the citric acid cycle and the electron transport chain. During the citric acid cycle, succinate is oxidized to fumarate, releasing electrons that are passed to FAD, resulting in the formation of FADH2. These electrons then enter the ETC at Complex II, where they are passed to the electron transport chain.

**Why Each Wrong Option is Incorrect**
* **Option A:** There is no Complex I in the ETC where FADH2 enters. Complex I is the first complex of the ETC, where NADH donates electrons.
* **Option B:** Complex IV, also known as Cytochrome Oxidase, is the final complex of the ETC, where oxygen is reduced to form water, and it is not the site of FADH2 entry.
* **Option C:** Complex III, also known as Cytochrome b-c1 Complex, is the third complex of the ETC, where electrons are passed from the QH2 pool to cytochrome c. It is not the site of FADH2 entry.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that FADH2 enters the ETC at Complex II, and not at Complex I, as NADH does. This distinction is crucial for understanding the electron transport chain and the process of oxidative phosphorylation.

**Correct Answer:** C. Complex II (Succinate Dehydrogenase)