A 61-year-old male with chronic schizophrenia managed on haloperidol 10 mg daily for fifteen years is referred by his primary care provider for evaluation of progressive motor symptoms including bilateral hand tremor, shuffling gait, and facial hypomimia developing over the past six months. His primary care provider is considering a referral to neurology for suspected Parkinson disease. The patient's psychiatric history is notable for multiple failed trials of atypical antipsychotics due to metabolic side effects and non-adherence. On examination, the tremor is present at rest and with sustained posture, is symmetric bilaterally, and has a frequency of approximately 6 Hz. Cogwheel rigidity is present in both upper extremities. There is no asymmetry of motor findings, and cognitive testing reveals stable baseline deficits consistent with chronic schizophrenia without progressive decline. Which assessment feature most strongly suggests drug-induced parkinsonism rather than idiopathic Parkinson disease?