skip to main content

Hypernatraemic dehydration physiology — MRCPCH TAS MCQ

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

HardPhysiologyHypernatraemic dehydration physiologyMRCPCH TAS

A 12-day-old breastfed infant has weight loss, lethargy and serum sodium of 158 mmol/L. The anterior fontanelle is slightly sunken and urine output is reduced. What is the most likely explanation?

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

Reveal the answer and explanation

Correct answer: B — Free water deficit exceeding sodium loss

Hypernatraemic dehydration occurs when free water loss or inadequate intake exceeds sodium loss, increasing extracellular osmolality. Primary sodium gain with water excess would not fit weight loss and reduced urine output. The pearl is that signs of dehydration may be less obvious because extracellular tonicity helps preserve intravascular volume.

Reference: Nelson Textbook of Pediatrics; Ganong's Review of Medical Physiology; RCPCH Theory examination syllabi