A 43-year-old client presents with complaints of excessive daytime sleepiness. She sometimes falls asleep without warning during meetings at work and while driving. She sometimes loses muscle tone suddenly if she laughs too hard or gets excited. Prior to prescribing medication, which diagnostic test should be completed?

Choice A reason: The symptoms described—excessive daytime sleepiness and cataplexy (sudden loss of muscle tone triggered by emotion)—are pathognomonic for narcolepsy type 1. Cerebrospinal fluid (CSF) analysis to measure hypocretin-1 (orexin-1) levels is a definitive diagnostic test. A level ≤ 110 pg/mL or one-third of the mean values in healthy subjects is diagnostic. Choice B reason: A complete blood count (CBC) is a routine laboratory test used to screen for infection, anemia, or other systemic conditions. While useful for general health screening, it provides no diagnostic value for sleep disorders or neurological conditions like narcolepsy and cataplexy, making it inappropriate as the primary diagnostic test for these specific symptoms. Choice C reason: Thyroid stimulating hormone (TSH) testing is often used to rule out hypothyroidism as a cause of fatigue or lethargy. However, hypothyroidism does not cause cataplexy. Given the specific clinical presentation of sleep attacks and emotion-triggered muscle weakness, TSH is an insufficient diagnostic tool for the suspected underlying neurological pathology. Choice D reason: A magnetic resonance imaging (MRI) of the brain is typically used to rule out structural abnormalities, tumors, or multiple sclerosis that might cause neurological symptoms. While it may be performed to exclude other differentials, it cannot diagnose narcolepsy, which is a functional neurochemical deficit in the hypothalamus that requires biochemical or sleep study validation.