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A 4-year-old boy presented with pallor, visual problems and recurrent multiple bilateral renal stones that had been unresponsive to (non-specific) medical treatment. He had seen a number of physicians over a period of two years without a diagnosis being made. He was found to have pancytopenia, the blood count showing: white cell count 3.1 × 109/l, hemoglobin concentration 86 g/l, and platelet count 102 × 109/l.

A bone marrow aspirate was performed to investigate the pancytopenia and revealed numerous clear cystine crystals, apparently free and within macrophages (images). This rare lysosomal storage disease is readily diagnosed from a bone marrow aspirate because of the presence of distinctive non-staining crystals. The crystals have straight edges and, when seen end on, are hexagonal (lower image).