Short‐term response to dietary therapy in molybdenum cofactor deficiency

Molybdenum cofactor deficiency was diagnosed in a 3‐month‐old girl who presented with microcephaly, developmental delay, severe irritability, and lactic acidosis. Dietary methionine restriction, with cysteine supplementation, was associated with moderate short‐term clinical improvement, including a resumption in predicted head growth, modest developmental progress, and a reduction in irritability. Clinical relapse was associated with noncompliance of dietary therapy 2 months later. Urinary sulfite levels measured by commercial dipsticks were useful in following therapy. Molybdenum cofactor deficiency is probably frequently underdiagnosed due to the lack of specific clinical or laboratory features. Screening of infants at risk for the presence of urinary sulfites or serum hypouricemia, or both, is both rapid and inexpensive.