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We report a 45-day-old male infant who presented with anuric renal failure and fluid overload due to urate nephropathy consequent upon hyperuricemia with hyperuricosuria. His maternal uncle had undergone renal transplantation for chronic renal failure secondary to uric acid nephrolithiasis. The levels of hypoxanthine phosphoribosyl transferase (HPRT) and adenine phosphoribosyl transferase activity in the baby were found to be quantitatively normal. However, when the HPRT activity was measured at low substrate concentrations [phosphoribosyl pyrophosphate (PRPP) and hypoxanthine] and compared with usual assay conditions, the HPRT activity at lower PRPP was less in the propositus, suggesting altered enzyme kinetics. Apparent K (m(PRPP)) and V (max), but not K (m(hypoxanthine)), were then found to be higher in the propositus than the control range. This is the first case of urate nephropathy secondary to altered enzyme kinetics presenting as early as 45 days. Uric acid nephropathy should be considered in the differential diagnosis of unexplained acute kidney injury in infants. In such cases, quantitative tests for HPRT enzyme activity may not be sufficient and altered enzyme kinetics should also be investigated.