No apparent cause for his limp was found.
He was referred to paediatric rheumatology. The history was probed further and it was apparent that he had pain worse on weight bearing. Examination of his feet showed a large verrucae on the sole of his foot - this was very tender.
The diagnosis was a verruca on his foot !
This case highlights the importance of careful clinical examination as the most crucial aspect of the evaluation; checking soles of the feet for easily missed problems and easily treatable benign problems is important. Verrucae, ingrowing toe nails, splinter / foreign body and even ill-fitting shoes are frequent causes of limp and should be considered before escalating investigations and especially if they are invasive, involve radiation or require sedation.
The photograph below shows the soles of the feet and surface anatomy.