A 28-month-old boy is admitted to the hospital having decreased appetite, vomiting, rapid breathing, and lethargy. Although he has only mild respiratory symptoms, he is breathing rapidly.

On physical examination, the child is lethargic and breathing at a rate of 56 breaths/min. There is no fever. He does not appear dysmorphic. His weight is at the 10th percentile, height at the 5th percentile, and head circumference at the 25th percentile. The remaining physical findings, including those on pulmonary evaluation, are normal.

His admission blood gas values are: pH, 7.03; P𝒸ℴ2, 12 torr; and base excess −26 mEq/L (normal, ±2 mEq/L). Blood chemistry findings are as follows: sodium, 137 mEq/L (137 mmol/L); chloride, 109 mEq/L (109 mmol/L); potassium. 4.2 mEq/L (4.2 mmol/L); bicarbonate, <5 mEq/L (5 mmol/L); blood urea nitrogen, 24 mg/dL (8.6 mmol/L); creatinine, 0.6 mg/dL (53 mcmol/L); anion gap, 23 (normal, 8 to 16); lactate, 0.7 mg/dL...

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