This patient has an asymmetric resting tremor associated with muscle rigidity, which is strongly suggestive of idiopathic Parkinson disease (PD).  Tremor is often the first manifestation of PD.  The tremor of PD characteristically occurs during rest and improves with activity.  It often occurs asymmetrically or involves one hand before the other.

There are several categories of medications that can be used for PD, including levodopa, dopamine agonists (eg, pramipexole), and anticholinergic medications (eg, trihexyphenidyl).  Medical therapy for PD is individualized.  Levodopa has potent antiparkinsonian effects and could be considered; it is associated with a risk of dyskinesias.

When tremor is the dominant symptom of PD (ie, without significant bradykinesia/rigidity), anticholinergic therapy (eg, trihexyphenidyl) is often effective.  Treatment is typically limited to younger patients (ie, ≤65), such as in this case, due to the risk of adverse effects in older patients.

(Choices A, C, and D)  Patients with essential tremor (ET) have tremors (usually of the hands), in contrast to this patient's tremor, that are decreased at rest, exacerbated by outstretched arms, and more pronounced at the end of goal-directed movements.  Propranolol and primidone are often used for ET.  Although clonazepam may also be effective for ET, other medications are preferred even in those patients due to the abuse potential of benzodiazepines.

(Choice B)  Clozapine, an atypical antipsychotic, is sometimes used to treat psychosis due to PD, but not tremor.

Educational objective:
Parkinson disease (PD) should be suspected in patients with an asymmetric resting tremor that is associated with rigidity or bradykinesia.  Trihexyphenidyl is an anticholinergic medication sometimes used in the treatment of PD, generally in younger patients where tremor is the primary symptom.