**Core Concept:** Parkinson's disease is a progressive neurological disorder characterized by the loss of dopaminergic neurons in the substantia nigra region of the brain. It leads to the deficiency of dopamine, a neurotransmitter essential for smooth movement and coordination.

**Why the Correct Answer is Right:** The correct answer is **C.** The first symptoms of Parkinson's disease typically involve motor symptoms, such as tremors, bradykinesia (slowness of movement), rigidity (stiffness), and postural instability (loss of balance). These motor symptoms arise from the loss of dopaminergic neurons in the substantia nigra, which results in dopamine deficiency.

**Why Each Wrong Option is Incorrect:**

A. Bradykinesia: Although bradykinesia is a primary symptom of Parkinson's disease, the correct answer focuses on the first symptom, which is tremors.
B. Tremors: While tremors are indeed a common symptom of Parkinson's disease, they are not the first symptom experienced by the patient.
D. Postural instability: Postural instability is a later symptom, not the first symptom experienced by the patient.

**Clinical Pearl:** The correct answer highlights the importance of understanding the sequence of symptoms in Parkinson's disease progression, which is crucial for early diagnosis and treatment. The motor symptoms typically develop in a "coast-down" pattern, starting with tremors and progressing to bradykinesia, rigidity, and postural instability. Timely recognition of these symptoms can significantly impact patient management, including the initiation of levodopa therapy to improve motor symptoms and manage complications like falls and fractures.

**Correct Answer:** C. Tremors