**Core Concept**
Parkinsonism is a clinical syndrome characterized by tremors, rigidity, bradykinesia, and postural instability, often resulting from degeneration of dopaminergic neurons in the substantia nigra. An oculogyric crisis is a rare but life-threatening complication of antipsychotic medication or parkinsonism treatment, involving involuntary, sustained eye movements.

**Why the Correct Answer is Right**
An oculogyric crisis in a patient with parkinsonism may be caused by the sudden withdrawal of anticholinergic medications or the initiation of dopamine agonists. This leads to an imbalance in dopamine and acetylcholine levels in the nigrostriatal pathway, resulting in abnormal eye movements. The patient's symptoms of trembling, dizziness, and upward reeling of eyes are consistent with an oculogyric crisis.

**Why Each Wrong Option is Incorrect**
**Option A:** A cerebellar infarct would typically present with ataxia, dysarthria, and nystagmus, but not necessarily upward reeling of the eyes.

**Option B:** A cholinergic crisis is a life-threatening condition caused by excessive acetylcholine activity, often due to organophosphate poisoning or overdose of anticholinesterase medications. It presents with muscarinic symptoms such as salivation, sweating, and bradycardia, rather than the described symptoms.

**Option C:** A cerebral infarct would typically present with focal neurological deficits, such as hemiparesis, aphasia, or visual field defects, rather than the described symptoms.

**Option D:** A cerebral infarct would not typically present with the described symptoms of oculogyric crisis.

**Clinical Pearl / High-Yield Fact**
Oculogyric crisis is a medical emergency that requires immediate treatment with anticholinergic medications or benztropine to prevent long-term damage to the nigrostriatal pathway.

**✓ Correct Answer: A. Oculygyric crisis**