A patient with a craniopharyngioma involving the hypothalamic-pituitary stalk develops diabetes insipidus. The posterior pituitary is intact on MRI. Which mechanism explains the DI?

• A Destruction of V2 receptors in the collecting duct
• B Interruption of axonal transport of AVP from supraoptic/paraventricular nuclei to the neurohypophysis ✓
• C Increased ADH metabolic clearance by the craniopharyngioma tissue
• D Compression of the renal medullary gradient by increased intra-abdominal pressure

Explanation

ADH (AVP) is synthesized in the supraoptic and paraventricular nuclei of the hypothalamus and transported along axons to terminals in the posterior pituitary for release. A pituitary stalk lesion (such as craniopharyngioma) interrupts this axonal transport, preventing AVP from reaching its release site despite intact posterior pituitary tissue. V2 receptors are renal and not affected by cranial lesions (option A); the tumor does not metabolize ADH (option C); option D is irrelevant to this mechanism.

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

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