Acid-Base Disorders USMLE Step 1 Practice Question

A 7-year-old boy presents to the emergency department with a 5-day history of severe watery diarrhea. His mother reports he has been increasingly lethargic and refusing to drink. On examination, he appears ill with weak, thready pulses and delayed capillary refill. Vital signs: temperature 37.1°C, heart rate 128/min, respiratory rate 28/min, blood pressure 88/52 mmHg. Laboratory studies show: pH 7.18 PCO2 18 mmHg HCO3− 7 mEq/L Na+ 128 mEq/L K+ 2.1 mEq/L Cl− 104 mEq/L BUN 48 mg/dL Creatinine 1.2 mg/dL Glucose 94 mg/dL Urine ketones: negative Which of the following best describes the primary acid-base disturbance?