skip to main content

Primary polydipsia — RACP Paediatrics MCQ

Instant feedback + full explanation. One question, done properly.

ModerateNephrologyPrimary polydipsiaRACP Paediatrics

A 4-year-old drinks large volumes of water (>3 L/day) and has polyuria. A water deprivation test shows serum osmolality increases to 292 mOsm/kg and urine concentrates to 750 mOsm/kg during water restriction. The child became distressed without access to water. What is the most likely diagnosis?

Educational content. Not a substitute for clinical judgement or local policy.

Reveal the answer and explanation

Correct answer: B — Primary polydipsia

Normal concentration of urine (>600-800 mOsm/kg) during water deprivation with a normal serum osmolality rules out diabetes insipidus and confirms primary (psychogenic/habitual) polydipsia. The kidneys can concentrate normally when stimulated. Behavioural management with gradual fluid restriction is the treatment. Rarely, compulsive water drinking may indicate an underlying psychological disorder.

Reference: RCH Melbourne – 2023 – DI CPG; RACP Paediatric Curriculum – Nephrology/Endocrine