A 6–month old boy weighing 3.2 kg presents with recurrent vomiting and polyuria. Investigations show blood area 60 mg/dl creatinine 0.7 mg/dL, calcium 12.8 mg/dL, phosphate 3 mg/dL, pH 7.45, bicarbonate 25 mEq/L and PTH 140 pg ml (normal <60 pg/ml). Daily urinary calcium excretion is reduced. Ultrasound abdomen show bilateral nephrocalcinosis. The most likely diagnosis is –