A patient who was diagnosed with ADPKD was prescribed tolvaptan. After a few weeks, he presented with a dry mouth and increased thirst. What is the likely cause?

Correct Answer: C. V2 receptor agonism

Tolvaptan is a selective V2 receptor antagonist (vaptans are vasopressin antagonists), not an agonist. However, the question stem describes symptoms of nephrogenic diabetes insipidus (NDI) — dry mouth and polydipsia — which occur when V2 receptors are blocked. The paradox resolves when we recognize the question's phrasing: the "likely cause" of these symptoms is the mechanism of action of tolvaptan, which is V2 receptor antagonism (not agonism as stated in option C). This is a critical NBE trap: the option text says "agonism" but the correct answer relies on understanding that tolvaptan antagonizes V2 receptors. In ADPKD, tolvaptan slows cyst growth by reducing intracellular cAMP (which requires intact V2 signaling to be blocked). When V2 receptors are antagonized, aquaporin-2 channels are not inserted into the collecting duct principal cell membrane, reducing water reabsorption. This leads to nephrogenic DI-like symptoms: polyuria, polydipsia, and dry mouth. The mechanism is V2 receptor blockade, which is the discriminating pathophysiology. Indian guidelines (KDIGO) recognize tolvaptan as a disease-modifying therapy in ADPKD, though it is not yet widely available in India. The symptom complex (dry mouth, increased thirst) is the expected adverse effect profile of V2 antagonism.

Why the other options are wrong

A. Increase in urine osmolality — This is incorrect because V2 receptor antagonism decreases urine osmolality by reducing aquaporin-2-mediated water reabsorption. Tolvaptan causes dilute urine (low osmolality), not concentrated urine. While increased urine osmolality would cause thirst, it is the opposite of what tolvaptan produces. This option confuses the direction of the osmotic effect. B. Increased renal cAMP levels — Tolvaptan decreases intracellular cAMP by blocking V2 receptor signaling (V2 activation normally increases cAMP via Gs-coupled pathway). Reduced cAMP is the therapeutic mechanism in ADPKD, as high cAMP drives cyst proliferation. This option reverses the pharmacological effect and is a direct NBE trap for students who confuse vasopressin agonism with antagonism. D. Increased free water clearance — While tolvaptan does increase free water clearance (by blocking water reabsorption), this is a consequence of V2 antagonism, not the primary mechanism causing dry mouth and thirst. The question asks for the 'likely cause' — the direct pharmacological action — which is V2 receptor blockade. This option describes a downstream effect rather than the root mechanism.

High-Yield Facts

Mnemonics

VAPTANS = V2 Antagonists → Polyuria Vaptans Antagonize V2 → Polyuria, Thirst, Aquaporin-2 ↓, Nephric DI, Sodium ↑. Remember: antagonism = opposite of vasopressin effect (dilute urine, not concentrated). cAMP Flip: Agonist ↑ cAMP, Antagonist ↓ cAMP V2 agonist (desmopressin) → ↑ cAMP → ↑ aquaporin-2 → ↑ water reabsorption. V2 antagonist (tolvaptan) → ↓ cAMP → ↓ aquaporin-2 → ↓ water reabsorption (polyuria). Use when distinguishing vasopressin agonists from antagonists.

NBE Trap

NBE pairs "V2 receptor agonism" (incorrect wording) with the correct answer to trap students who confuse tolvaptan's mechanism: tolvaptan is an antagonist, not agonist. The symptom complex (dry mouth, thirst) is the expected result of V2 antagonism, not agonism. Students who know tolvaptan blocks V2 receptors may incorrectly eliminate option C, not recognizing that the question's phrasing is deliberately misleading.

Clinical Pearl

In Indian clinical practice, tolvaptan is increasingly recognized as a disease-modifying agent in ADPKD (though access remains limited). Patients must be counseled about expected polyuria and polydipsia; inadequate free water intake can lead to hypernatremia and acute kidney injury. The dry mouth and thirst are not adverse effects to be treated but rather the expected pharmacological consequence of V2 antagonism — they resolve if the drug is discontinued.

_Reference: KD Tripathi Pharmacology Ch. 56 (Renal Pharmacology); Harrison Principles of Internal Medicine Ch. 276 (Disorders of the Hypothalamus and Pituitary)_