**Core Concept**
Mitochondrial membranes are composed of a unique set of lipids that are essential for the proper functioning of these organelles. Among these lipids, one type plays a crucial role in maintaining the integrity and fluidity of the mitochondrial membrane.

**Why the Correct Answer is Right**
The correct answer is Cardiolipin, which is a major lipid component of the mitochondrial inner membrane. Cardiolipin is a phospholipid with a unique structure, consisting of two phosphatidic acid groups linked by a glycerol backbone. This unique structure allows cardiolipin to play a crucial role in maintaining the fluidity and integrity of the mitochondrial membrane. Additionally, cardiolipin is involved in the regulation of mitochondrial membrane dynamics and the proper functioning of the electron transport chain.

**Why Each Wrong Option is Incorrect**
**Option A:** Phosphatidylserine is a phospholipid component of the plasma membrane, but it is not a major lipid of the mitochondrial membrane.
**Option B:** Lecithin is a generic term for a group of phospholipids, including phosphatidylcholine, but it is not a specific lipid component of the mitochondrial membrane.
**Option C:** Cholesterol is a lipid component of many cellular membranes, but it is not a major lipid of the mitochondrial membrane.

**Clinical Pearl / High-Yield Fact**
Cardiolipin is essential for maintaining the proper functioning of the electron transport chain and for regulating mitochondrial membrane dynamics. Deficiencies in cardiolipin have been implicated in various mitochondrial disorders, including Barth syndrome.

**Correct Answer:** C. Cardiolipin