A 1-month-old infant with poor feeding and intermittent seizures is found to have a serum calcium of 6.2 mg/dL and a serum magnesium of 0.8 mg/dL. An intact parathyroid hormone (PTH) level is inappropriately low for the degree of hypocalcemia. Administration of intravenous calcium provides only transient improvement. What is the most likely underlying mechanism for this infant’s condition?