Arlo is the family’s first child, born in a non-consanguineous marriage. His perinatal history was uneventful, and he was born at full term with a birth weight of 3.2 kg. Arlo first attended the Paediatric clinic for poor weight gain at 17 months old. Physical examination showed his body height and weight were at the third centile. Initial screening tests, including complete blood counts, liver enzymes, urea, creatinine, electrolytes and thyroid function tests, were normal. Arlo’s mother described him as a picky eater. He has his weight gain along the third centile, but her body height dropped off during the follow-up. What could be the underlying cause of Arlo’s failure to thrive? His case reveals a story full of unexpected findings.