1: Presentation and Early Diagnostic Approach

A 6-month-old infant is referred for failure to thrive and recurrent vomiting. The parents report that the child drinks more milk than usual but vomits frequently after feeds. The infant has not gained weight since 4 months and has persistent irritability. Examination shows poor muscle tone and frontal bossing. Laboratory results are: Serum bicarbonate: 14 mmol/L (normal: 22–28 mmol/L) Serum potassium: 2.9 mmol/L (normal: 3.5–5.0 mmol/L) Urine dipstick: glycosuria (++), proteinuria (+) The infant’s mother has no significant past medical history, but the father’s cousin had a kidney transplant as a child.

What is the next best diagnostic step?