Cystinuria and mental deficiency.

Several studies have shown that the incidence of cystinuria in mentally retarded populations is higher than it is in the general population. The present study found that a group of cystinurics did not differ significantly in their intelligence from their normal siblings or intelligence test norms. Since cystinuria is a heterogeneous disease, the findings of the present study could be reconciled with previous findings by supposing that only certain genotypes for cystinuria lead to mental retardation.