Which of the following medications is specifically effective against motion sickness?

Scopolamine is a medication that is specifically effective against motion sickness due to its anticholinergic properties. It works by blocking the action of acetylcholine in the vestibular system of the inner ear, which is responsible for balance and spatial orientation. When the body experiences motion that conflicts with visual inputs, such as during travel, this can trigger symptoms of nausea and dizziness. By inhibiting these signals, scopolamine effectively prevents the onset of motion sickness.

The other medications listed are not primarily used for this indication. Metoclopramide, for example, is a dopamine antagonist that can be effective for nausea and vomiting related to various causes, including gastroparesis, but not specifically for motion sickness. Ondansetron is a serotonin 5-HT3 receptor antagonist used mainly in the prevention of nausea and vomiting associated with chemotherapy and postoperative states, rather than for motion sickness. Dexamethasone is a corticosteroid that can be helpful as an antiemetic in certain scenarios, including chemotherapy-induced nausea, but again, it does not target the mechanisms of motion sickness directly. Therefore, scopolamine is clearly the appropriate choice for preventing motion sickness.