Movement Disorders USMLE Step 1 Practice Question
A 51-year-old man with a 12-year history of Parkinson disease presents to clinic complaining of unpredictable episodes where his medications "wear off" within 2-3 hours of taking them, alternating with periods of involuntary writhing movements of his limbs during peak drug effect. He reports these fluctuations have worsened over the past 2 years despite increasing his carbidopa-levodopa dose to five times daily. Examination during an "on" period reveals prominent dyskinesias. Brain MRI shows no structural abnormalities. Which of the following best explains the development of both motor fluctuations and dyskinesias in this patient?
Answer choices
- ADepletion of striatal dopamine storage capacity due to progressive loss of substantia nigra neurons
- BPulsatile rather than continuous dopaminergic stimulation of supersensitive striatal receptorsCorrect answer
- CAccumulation of levodopa metabolites that competitively inhibit dopamine receptor binding
- DDevelopment of antibodies against dopamine transporter proteins reducing medication efficacy
- EDownregulation of dopamine receptors from chronic high-dose levodopa exposure
- FConversion of levodopa to serotonin in non-dopaminergic neurons causing motor side effects
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