A physician-validated, board-style question from the Active Transport QBank. Try it, then check the reasoning for every option.
A 56-year-old male comes to the physician because of a 2-month history of excessive sleepiness. He reports that he has been sleeping for an average of 10 to 12 hours at night and needs to take multiple naps during the day. Six months ago, he was diagnosed with small cell lung carcinoma and underwent prophylactic cranial irradiation. This patient's symptoms are most likely caused by damage to which of the following structures?
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A
Ventromedial nucleusIncorrect. Ventromedial nucleus damage causes hyperphagia and obesity (loss of satiety), not hypersomnolence.
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B
Suprachiasmatic nucleusCorrect. Suprachiasmatic nucleus governs circadian rhythm and sleep-wake cycles; damage causes hypersomnolence and circadian disruption.
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C
Supraoptic nucleusIncorrect. Supraoptic nucleus produces ADH/vasopressin; damage causes central diabetes insipidus (polyuria, polydipsia), not sleepiness.
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D
Subthalamic nucleusIncorrect. Subthalamic nucleus is part of the basal ganglia motor circuit; lesions cause hemiballismus, not sleep disturbances.
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E
Paraventricular nucleusIncorrect. The paraventricular nucleus produces oxytocin and CRH and helps regulate the HPA axis; damage causes neuroendocrine deficits rather than the hypersomnolence and circadian disruption that follow SCN injury.
↑ Tap an answer to reveal the reasoning
Answer: B. Excessive daytime sleepiness with prolonged nighttime sleep developing after cranial irradiation suggests damage to a hypothalamic structure regulating sleep-wake cycles. The suprachiasmatic nucleus (SCN) of the hypothalamus is the master circadian pacemaker, receiving direct retinal input via the retinohypothalamic tract (entraining the circadian rhythm to light-dark cycles) and orchestrating the timing of sleep, wakefulness, hormone secretion, and many other processes through projections to the pineal gland (regulating melatonin) and other CNS structures.
Prophylactic cranial irradiation (PCI) for small cell lung cancer can damage hypothalamic structures, disrupting circadian regulation and producing hypersomnolence and altered sleep-wake patterns.
Key hypothalamic nuclei to distinguish:
- Suprachiasmatic: circadian rhythm/sleep-wake (CORRECT here).
- Ventromedial: satiety; destruction causes hyperphagia and obesity.
- Supraoptic: vasopressin synthesis; destruction causes central diabetes insipidus.
- Subthalamic: motor control (basal ganglia, not hypothalamus); lesions cause hemiballismus.
**Why each option:**
**A.** Ventromedial nucleus damage causes hyperphagia and obesity (loss of satiety), not hypersomnolence.
**B.** Correct. Suprachiasmatic nucleus governs circadian rhythm and sleep-wake cycles; damage causes hypersomnolence and circadian disruption.
**C.** Supraoptic nucleus produces ADH/vasopressin; damage causes central diabetes insipidus (polyuria, polydipsia), not sleepiness.
**D.** Subthalamic nucleus is part of the basal ganglia motor circuit; lesions cause hemiballismus, not sleep disturbances.