**Core Concept**
The patient's symptoms suggest a diagnosis of Parkinson's disease, a neurodegenerative disorder characterized by the degeneration of dopaminergic neurons in the substantia nigra, leading to a decrease in dopamine production. This results in the characteristic motor symptoms of Parkinson's disease, including resting tremor, rigidity, and bradykinesia.

**Why the Correct Answer is Right**
Dopamine plays a crucial role in motor control, and its deficiency leads to the motor symptoms of Parkinson's disease. The substantia nigra pars compacta, the primary site of dopamine production, is affected in Parkinson's disease. The decrease in dopamine production is thought to be due to the loss of dopaminergic neurons and the reduction in dopamine synthesis. This decrease in dopamine results in the characteristic motor symptoms of Parkinson's disease, including resting tremor, rigidity, and bradykinesia.

**Why Each Wrong Option is Incorrect**
**Option A:** Glucose metabolism is not directly related to the motor symptoms of Parkinson's disease.
**Option B:** Acetylcholine is actually increased in Parkinson's disease, which can contribute to the development of tremors and other motor symptoms.
**Option C:** GABA (gamma-aminobutyric acid) is an inhibitory neurotransmitter that is not directly related to the motor symptoms of Parkinson's disease.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that Parkinson's disease is a neurodegenerative disorder that affects the substantia nigra, leading to a decrease in dopamine production and the characteristic motor symptoms.

**Correct Answer:** D. Dopamine.