Which antiemetics are contraindicated in patients with Parkinson's disease due to their mechanism of action?

The correct choice highlights the medications metoclopramide and prochlorperazine, which are both dopamine antagonists. In patients with Parkinson's disease, the use of dopamine antagonists is particularly problematic because these medications work by blocking dopamine receptors in the brain. Since Parkinson's disease is characterized by a deficiency of dopamine due to the degeneration of dopaminergic neurons, using medications that further block dopamine can exacerbate motor symptoms and lead to increased rigidity and bradykinesia in these patients.

In contrast, the other options contain antiemetics that may not have the same direct detrimental effect on the dopamine pathways. Cyclizine and dimenhydrinate are primarily antihistamines with limited impact on dopamine receptors, while ondansetron and granisetron are serotonin receptor antagonists that generally do not contribute to worsening Parkinsonian symptoms. Promethazine and scopolamine, while they can have some central nervous system effects, do not pose the same level of risk as metoclopramide and prochlorperazine due to their different mechanisms of action. Thus, metoclopramide and prochlorperazine are specifically contraindicated in patients with Parkinson's disease due to their potential to exacerbate the condition.