## **Core Concept**
Parkinson's disease (PD) is a neurodegenerative disorder characterized by the depletion of dopamine-producing neurons in the substantia nigra, leading to motor and non-motor symptoms. Key motor features include tremors, rigidity, bradykinesia, and postural instability. The question tests the understanding of specific clinical manifestations associated with PD.

## **Why the Correct Answer is Right**
The correct answer includes features commonly observed in Parkinson's disease:
- **Tremor**: A classic symptom, often presenting as a resting tremor.
- **On-off phenomenon**: A complication of long-term levodopa therapy, characterized by fluctuations in response to medication.
- **Decreased Blinking**: Patients with PD often exhibit reduced blinking, contributing to dry eye symptoms.
The combination of these features accurately reflects the clinical presentation of PD.

## **Why Each Wrong Option is Incorrect**
- **Option B: Spasticity** - While rigidity is a hallmark of PD, spasticity (increased muscle tone leading to stiffness and difficulty moving) is more characteristic of upper motor neuron lesions, not typically a primary feature of PD.
- **Option D: Objective ↓ muscle power** - Muscle weakness can occur in PD but is not a defining feature; the primary motor symptoms are tremor, rigidity, bradykinesia, and postural instability.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that **Parkinson's disease is primarily diagnosed based on clinical features**, as there are no definitive diagnostic tests. The presence of bradykinesia along with either tremor or rigidity is highly suggestive of PD.

## **Correct Answer:** C. a, c, e