**Core Concept**
Parkinsonism is a neurodegenerative disorder characterized by the degeneration of dopamine-producing neurons in the substantia nigra, leading to a decrease in dopamine levels in the striatum. This results in a range of motor symptoms, including tremors, rigidity, bradykinesia, and postural instability.

**Why the Correct Answer is Right**
The earliest symptom of Parkinsonism is often a subtle, unilateral tremor, typically manifesting as a resting tremor. This tremor is thought to result from the abnormal activity of the subthalamic nucleus and the globus pallidus, which are part of the basal ganglia circuitry. The decreased dopamine levels in the striatum disrupt the normal balance between excitatory and inhibitory neurons, leading to the characteristic tremor.

**Why Each Wrong Option is Incorrect**
**Option A:** Bradykinesia and postural instability are common symptoms of Parkinsonism, but they are not typically the earliest manifestations of the disease. Bradykinesia refers to the slowing of movement, while postural instability refers to a loss of balance and coordination.
**Option B:** Rigidity, or increased muscle tone, is a hallmark symptom of Parkinsonism, but it usually develops later in the course of the disease.
**Option C:** Dysautonomia, or autonomic dysfunction, can occur in Parkinsonism, but it is not the earliest symptom.

**Clinical Pearl / High-Yield Fact**
The "4 As" of Parkinsonism - akinesia (bradykinesia), akathisia (restlessness), anhydrosis (decreased sweating), and autonomic dysfunction - are important to recognize in the early stages of the disease.

**Correct Answer: C. Dysautonomia.