A patient undergoes transphenoidal resection of a large pituitary macroadenoma. Postoperatively he develops polyuria (urine output 6 L/day) with dilute urine (specific gravity 1.002) and hypernatremia. Which of the following correctly describes the hormonal defect?

• A Loss of oxytocin-secreting neurons in the paraventricular nucleus
• B Destruction of the posterior pituitary neurohypophysis only, irreversible diabetes insipidus
• C Deficiency of pituitary aquaporin-2 channels reducing urine concentrating ability
• D Damage to magnocellular neurons in the supraoptic and paraventricular nuclei ✓

Explanation

ADH (vasopressin) is synthesized in magnocellular neurons of the supraoptic nucleus (predominantly) and paraventricular nucleus, then transported to the posterior pituitary for release. Surgical damage to these hypothalamic nuclei or the pituitary stalk above the median eminence causes permanent central DI. Destruction of only the posterior pituitary is often followed by recovery because residual axon stumps at the median eminence continue ADH release. Aquaporin-2 channels are in the collecting duct (kidney), not pituitary. Oxytocin loss does not produce DI.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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