**Core Concept**
Hyperammonemia affects the citric acid cycle, also known as the Krebs cycle or tricarboxylic acid (TCA) cycle, by interfering with a crucial step. The **citric acid cycle** is a key metabolic pathway that generates energy through the oxidation of acetate derived from carbohydrates, fats, and proteins into carbon dioxide and water.

**Why the Correct Answer is Right**
The correct answer is related to the depletion of **alpha-ketoglutarate**, a critical intermediate in the citric acid cycle. Hyperammonemia leads to the formation of **glutamine** from **alpha-ketoglutarate** and **ammonia** via the enzyme **glutamine synthetase**. This reaction depletes **alpha-ketoglutarate**, thereby impairing the citric acid cycle.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because while NAD+ is crucial for the citric acid cycle, hyperammonemia does not directly deplete it.
**Option B:** Incorrect as FAD is another essential cofactor but not directly depleted by hyperammonemia.
**Option C:** Incorrect because although CoA is vital for the cycle, it's not the direct target of hyperammonemia's effect.

**Clinical Pearl / High-Yield Fact**
Hyperammonemia can lead to severe neurological symptoms due to its toxic effects on the brain. Remember that **alpha-ketoglutarate** depletion is a key mechanism by which hyperammonemia impairs energy production in the brain and other tissues.

**Correct Answer:** D. alpha-ketoglutarate