A 19-day-old boy has vomited after most feeds for the past week. He is 9 per cent below his birth weight, lethargic, with dry mucous membranes and cool peripheries. His scrotum is darkly pigmented. He was born at term after an uncomplicated pregnancy and was well at his newborn check. Venous blood has been sent to the laboratory.
Which one of the following plasma results would favour aldosterone deficiency over pyloric stenosis as the cause of this baby's dehydration?