Calcium, magnesium and phosphate disorders — OSCE

Candidate brief

You are the paediatric registrar. The child is 4 years old and has had bowing of the legs and slow growth for over a year. The team has confirmed X-linked hypophosphataemic rickets, an inherited condition. The parents want to understand what it is, whether it can be cured, why his phosphate is low when his other bones tests look normal, what the treatment involves, and what to watch for. Explain clearly, check understanding, and agree a plan. [9]

Marking domains

Clinical knowledge and accuracy (3). Explains that X-linked hypophosphataemic rickets is an inherited bone condition caused by an excess of a hormone called fibroblast growth factor 23 that makes the kidneys waste phosphate, so the phosphate is low while the calcium and parathyroid hormone stay normal; that it is lifelong but treatable, with a good outlook for growth and mobility; and that the modern treatment is burosumab, an injection that blocks the hormone and lets the kidneys keep phosphate. [9][10]

Communication and plain language (3). Avoids jargon or defines it; uses an analogy (the kidney is a "sieve" and the hormone is "leaving the sieve open"); paces the information; pauses to check understanding; invites questions. Conveys that it is genetic and X-linked, usually inherited from the mother, and does not imply anything was done wrong. [9]

Management plan and safety-net (3). Explains the burosumab injection schedule, the need for regular blood tests to monitor phosphate, calcium and kidney function, monitoring of growth and leg alignment, and possible orthopaedic input for the bowing. Gives a clear safety-net: seek help for severe bone pain, worsening bowing, dental problems or abscesses, limp, or any new symptom; flag the condition to any new doctor or dentist. [10]

Empathy and partnership (1). Acknowledges the family's concern about growth and appearance, validates their worry, and frames the diagnosis positively as a named, treatable condition with a clear plan and good prognosis. [9]

Examiner notes

Strong candidates explain WHY the phosphate is low while the calcium is normal (the FGF23 hormone selectively wastes phosphate), name burosumab as the modern standard and what it does, and give a concrete safety-net rather than vague reassurance. Weak candidates call it "just rickets," confuse it with vitamin D deficiency, or fail to mention the FGF23 mechanism or the injection treatment. [10][9]