Mucopolysaccharidosis type I disguised as rickets.

A 16-year-old boy with widening of the large joints of the extremities and bilateral genu valgum had been extensively treated with oral vitamin D, with little clinical benefit. A diagnosis of vitamin D-resistant rickets was considered initially but a thorough clinical examination and skeletal survey was suggestive of mucopolysaccharidosis. The diagnosis was confirmed biochemically and subtype classification pointed toward the type I variety of the storage disorder. Absence of mental retardation is very unusual in mucopolysaccharidosis type I, which itself is an uncommon clinical entity. This particular disease can be misdiagnosed as vitamin D-resistant rickets in the absence of thorough systemic examination and an attentive look at the skeletal surveys. Spondyloepiphyseal dysplasia is another close differential of mucopolysaccharidosis and it should be ruled out in all cases of suspected spondyloepiphyseal dysplasia.