Nephrolithiasis and nephrocalcinosis — OSCE

Candidate brief

You are the paediatric registrar. The patient is 13 and has passed his second calcium oxalate kidney stone in two years, this time after an episode of colicky pain. His metabolic workup has confirmed idiopathic hypercalciuria with an otherwise normal blood panel. The consultant has asked you to explain to his parents why the stones keep recurring, what can be done to stop them, and what a thiazide diuretic would add. The parents are worried this will damage his kidneys and want to know if it is curable. Explain clearly, check their understanding, and agree a plan. [1]

Marking domains

Clinical knowledge and accuracy (3). Explains that their son's kidneys are excreting too much calcium into the urine (hypercalciuria), which makes the urine supersaturated so that calcium and oxalate crystallise into stones; that this is a metabolic tendency rather than damage to the kidneys; that without prevention about half of children form another stone within a few years; and that a prevention plan sharply lowers that risk. Names the pillars: high fluid intake to keep urine dilute, a normal (not low) calcium diet with reduced salt and oxalate, and a thiazide diuretic to reduce renal calcium excretion if fluids and diet alone are insufficient. [1][11]

Communication and plain language (3). Avoids jargon or defines it; uses an analogy (the urine is too concentrated, like sugar settling at the bottom of a glass); gives a concrete fluid target (enough to keep the urine pale and abundant, about two litres a day in an adolescent); corrects the likely misconception that he should avoid calcium (a low-calcium diet paradoxically increases stones by raising oxalate absorption). Paces the information, pauses to check understanding, and invites questions. [1][5]

Management plan and safety-net (3). Lays out the daily plan: carry water and drink through the day, reduce salt and high-oxalate foods, keep normal calcium (dairy with meals), and a follow-up urine test to check the calcium is falling. Explains that a thiazide diuretic is a small daily tablet that tells the kidney to hold on to calcium, used if fluids and diet do not control the hypercalciuria, with monitoring of blood tests. Gives a clear safety-net: seek help for severe colicky pain, fever with flank pain (which could mean an infected obstructed stone), visible haematuria, or reduced urine output. [10][11]

Empathy and partnership (1). Acknowledges the parents' worry about kidney damage, reassures them that idiopathic hypercalciuria does not itself harm the kidneys when stones are prevented, and frames the plan as a partnership the family owns day to day. [1]

Examiner notes

Strong candidates explain WHY a normal-calcium (not low-calcium) diet is correct, give a concrete fluid target rather than "drink more", and name thiazide's mechanism (reduce renal calcium excretion) in plain terms. Weak candidates tell the family to avoid calcium, omit the safety-net for an infected obstructed stone, or offer a cure. The metabolic workup rationale — that a first or second stone in a child deserves evaluation because recurrence is the rule and a metabolic or genetic cause underlies the majority — is a discriminator at the high end. [3] [11]