Polyuria and polydipsia — OSCE

Candidate brief

You are the paediatric registrar. The patient is a 4-month-old boy admitted with poor weight gain, irritability and hypernatraemic dehydration. Investigations have confirmed congenital nephrogenic diabetes insipidus: his blood glucose is normal, his serum sodium was 158 mmol per litre, his urine osmolality was maximally dilute, and he did not respond to desmopressin. The consultant has asked you to explain to his parents what the diagnosis means, why the usual hormone tablet will not work for him, what the daily care involves, and how to keep him safe. The parents are frightened that his kidneys are failing and that he will not grow. Explain clearly, check their understanding, and agree a plan. [9]

Marking domains

Clinical knowledge and accuracy (3). Explains that their son's kidneys cannot respond to the hormone (vasopressin) that normally tells the body to hold on to water, so he passes large volumes of dilute urine and loses too much water; that this is a problem with how the kidney responds, not kidney failure, and that his kidneys filter blood normally. Names that desmopressin — the usual treatment — will not work because the defect is in the kidney's response, not a lack of hormone. States the day-to-day plan: give him free access to water and feeds around the clock (a nasogastric drip overnight if needed to protect growth), keep his diet low in salt and not over-high in protein to reduce the water his body has to lose, and a small daily thiazide tablet (with amiloride or indometacin) that paradoxically reduces the urine volume. [9][3]

Communication and plain language (3). Avoids jargon or defines it; uses an analogy (his kidneys have their hands over their ears and cannot hear the message to save water); reassures the parents that his kidneys are not failing — they are just not listening to the water-saving signal. Corrects the likely misconception that restricting his fluids will help — he must drink freely to replace what he loses. Paces the information, pauses to check understanding, and invites questions about feeding, growth and family implications. [3][11]

Management plan and safety-net (3). Lays out the daily plan: feed and offer water regularly through the day and night, accept the wet nappies as expected, attend follow-up to monitor weight and the blood sodium, and give the thiazide consistently. Explains that genetic testing will confirm the inherited form and is relevant because the condition can run in families. Gives a clear safety-net: seek urgent help if he has fewer wet nappies, is drowsy or floppy, has a fever with reduced drinking, is irritable or vomiting — because he can become dangerously dehydrated quickly, especially with intercurrent illness. [12][9]

Empathy and partnership (1). Acknowledges the parents' fear about his kidneys and growth, reassures them that with consistent water, diet and medication most children grow and develop normally, and frames the plan as a partnership the family owns day to day with the team's support. [3]

Examiner notes

Strong candidates explain WHY desmopressin does not work in nephrogenic disease (the defect is downstream of the V2 receptor), give a concrete daily plan centred on free water and a low-solute diet rather than a drug, and name the paradoxical thiazide mechanism in plain terms. Weak candidates tell the family to restrict fluids, promise that desmopressin will fix it, or omit the safety-net for rapid dehydration during illness. The distinction between nephrogenic disease (kidney cannot respond) and kidney failure (kidney cannot filter) is a key discriminator at the high end, as is the recognition that an infant cannot defend their own water balance and depends entirely on the caregivers. [9] [11]