**Core Concept**
Dextromethorphan is a non-opioid cough suppressant with complex pharmacological properties, including actions as a serotonin and norepinephrine reuptake inhibitor, sigma receptor agonist, and NMDA receptor antagonist. It is known for its potential to produce psychoactive effects, particularly at higher doses.

**Why the Correct Answer is Right**
Dextromethorphan's NMDA receptor antagonism is thought to contribute to its hallucinogenic properties. At therapeutic doses, dextromethorphan is primarily metabolized by the cytochrome P450 2D6 enzyme to its active metabolite, dextrorphan, which also exhibits NMDA receptor antagonism. This mechanism is similar to that of other dissociative anesthetics, such as ketamine and phencyclidine, which are known to produce hallucinations.

**Why Each Wrong Option is Incorrect**
**Option A:** Morphine is an opioid analgesic that can produce euphoria and dependence but is not typically associated with hallucinations.
**Option C:** Diphenhydramine is an antihistamine with sedative and anticholinergic effects but is not known for causing hallucinations.
**Option D:** Aprepitant is a substance P/neurokinin 1 receptor antagonist used primarily for preventing chemotherapy-induced nausea and vomiting; it is not associated with hallucinations.

**Clinical Pearl / High-Yield Fact**
Dextromethorphan can produce a range of psychoactive effects, including euphoria, dissociation, and hallucinations, particularly when taken in large doses or combined with other substances that inhibit its metabolism, such as SSRIs.

**✓ Correct Answer: B. Dextromethorphan**