In suspected pheochromocytoma, which mechanism explains water retention related to the tumor?

• A. Catecholamine deficiency reduces renal perfusion
• B. Vasoconstriction of renal arteries triggers the renin-angiotensin system, increasing water reabsorption
• C. Increased aldosterone drives sodium loss
• D. Decreased ADH causes dehydration

Answer: (B)

Excess catecholamines from pheochromocytoma cause widespread vasoconstriction, including in the renal arteries, which lowers renal blood flow. The kidneys sense this reduced perfusion and release renin, activating the renin-angiotensin-aldosterone system. Angiotensin II helps maintain glomerular filtration by constricting the efferent arteriole and stimulates aldosterone release. Aldosterone increases sodium reabsorption in the distal nephron, and water follows sodium, leading to fluid retention. This sequence explains why water retention occurs in the context of a pheochromocytoma.

In this scenario, water retention isn’t due to catecholamine deficiency, aldosterone driving sodium loss, or decreased ADH causing dehydration, so those mechanisms don’t fit as explanations for the observed fluid retention.