**Core Concept**
Huntington's disease is a neurodegenerative disorder characterized by the progressive loss of motor control, cognitive decline, and psychiatric problems. It is caused by an expansion of a CAG repeat in the huntingtin gene, leading to a toxic protein that causes neuronal damage.

**Why the Correct Answer is Right**
The correct answer is **Glutamate**. The loss of glutamate is not the direct cause of Huntington's disease, but rather the disease is characterized by an imbalance in excitatory and inhibitory neurotransmission. The mutant huntingtin protein disrupts normal neuronal function, leading to an overactivation of glutamate receptors and excitotoxicity. This results in the death of medium spiny neurons in the striatum, a key region affected in Huntington's disease.

**Why Each Wrong Option is Incorrect**
**Option A:** **Dopamine** is not directly related to the pathophysiology of Huntington's disease, although dopamine release is affected in the disease due to the loss of striatal neurons.
**Option B:** **Serotonin** is not directly implicated in the pathophysiology of Huntington's disease, although serotonin receptors may be affected in the disease due to the widespread neuronal damage.
**Option C:** **GABA** is an inhibitory neurotransmitter, and while its release is affected in the disease, the disease is characterized by an imbalance in excitatory and inhibitory neurotransmission, with an overactivation of glutamate receptors.

**Clinical Pearl / High-Yield Fact**
The expansion of the CAG repeat in the huntingtin gene is an autosomal dominant trait, meaning that a single copy of the mutated gene is sufficient to cause the disease.

**Correct Answer:** Glutamate