What condition is indicated by a mousy or musty odor of urine?

A mousy or musty odor of urine is indicative of phenylketonuria (PKU), which is a metabolic disorder caused by the lack of an enzyme called phenylalanine hydroxylase. In individuals with PKU, phenylalanine accumulates in the body due to this enzymatic deficiency. The excess phenylalanine is converted into phenylacetate, which contributes to the characteristic musty or mousy odor of the urine. This specific odor is a key clinical feature that may lead to the diagnosis of this condition, particularly when a patient presents with developmental delays or other neurological symptoms.

Other conditions, such as diabetes, can produce sweet-smelling urine due to ketosis or the presence of glucose, while urinary tract infections typically lead to foul-smelling urine due to the presence of bacteria and metabolic byproducts. Kidney stones may cause hematuria or pain but do not correlate with a musty odor in urine. Thus, the unique musty scent associated with phenylketonuria clearly sets it apart as the correct answer.