**Core Concept**
Narcolepsy is a chronic neurological disorder characterized by excessive daytime sleepiness, sudden attacks of sleep, and other symptoms due to the loss of hypocretin/orexin neurons in the hypothalamus.

**Why the Correct Answer is Right**
The symptoms of narcolepsy are primarily due to the disruption of the normal sleep-wake cycle, leading to excessive daytime sleepiness, cataplexy, sleep paralysis, and hypnagogic hallucinations. These symptoms result from the loss of hypocretin/orexin neurons, which play a crucial role in regulating arousal and wakefulness.

**Why Each Wrong Option is Incorrect**
**Option A:** Not a symptom of narcolepsy.
**Option B:** Cataplexy, a sudden loss of muscle tone, is a hallmark symptom of narcolepsy, making this option incorrect.
**Option C:** Sleep paralysis is a symptom of narcolepsy, characterized by the temporary inability to move or speak while falling asleep or waking up.
**Option D:** Hypnagogic hallucinations are vivid, dream-like experiences that occur when falling asleep or waking up, and are a symptom of narcolepsy.

**Clinical Pearl / High-Yield Fact**
Narcolepsy often presents with a combination of excessive daytime sleepiness, cataplexy, sleep paralysis, and hypnagogic hallucinations, which can be challenging to diagnose due to their variable presentation.

**Correct Answer: B. Cataplexy is a hallmark symptom of narcolepsy.**