An 8–day old breast–fed baby presents with vomiting, poor feeding and loose stools. On examination the heart rate is 190/minute, blood pressure 50/30 mmHg, respiratory rate 72 breaths/minute and capillary refill time of 4 seconds. Investigations show hemoglobin level of 15 g/dl. Na 120 mEq/l, K 6.8 mEq/l, Cl 81 meq/l, bicarbonate 15 mEq/l, urea 30 mg/dl and creatinine 0.6 mg/dl. the most likely diagnosis is –