A 34-year-old woman comes to the office due to a 3-day history of watery diarrhea. The patient has no fever. She has no other medical conditions and takes no medications. Blood pressure is 120/80 mm Hg and pulse is 90/min. Examination shows mild abdominal discomfort but is otherwise normal. Compared to normal laboratory values, which of the following findings are most likely present in this patient?

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11ec160b_3cfb_f84a_9249_ffc72752387d_MD0001_00 This patient with several days of diarrhea is expected to have primary metabolic acidosis, with low blood pH (<7.35), low serum bicarbonate (HCO₃⁻) (<24 mEq/L), and compensatory low partial pressure of carbon dioxide in arterial blood (PaCO₂) (<40 mm Hg). Severe diarrhea involves substantial loss of HCO₃⁻ in the stool, leading to nonanion gap metabolic acidosis. The reduced blood pH increases ventilatory drive to facilitate CO₂ removal by the lungs, creating a compensatory respiratory alkalosis. (Choices B and C) Low pH with high PaCO₂ is consistent with respiratory acidosis. Full metabolic compensation via HCO₃⁻ reabsorption by the kidneys requires approximately 72 hours; therefore, HCO₃⁻ is high in chronic respiratory acidosis (eg, CO₂ retention due to chronic obstructive pulmonary disease) and near normal in acute respiratory acidosis (eg, suppressed respiratory drive due to opioid overdose). (Choice D) High pH with low PaCO₂ represents respiratory alkalosis, as occurs with hyperventilation at high altitude. HCO₃⁻ is low due to compensatory metabolic acidosis. (Choice E) High pH with high HCO₃⁻ represents metabolic alkalosis, as occurs with the loss of H⁺ with severe vomiting. PaCO₂ is high due to compensatory respiratory acidosis. Educational objective: Severe diarrhea causes substantial loss of bicarbonate (HCO₃⁻) in the stool and is a common cause of nonanion gap metabolic acidosis. Low blood pH (<7.35) and low serum HCO₃⁻ (<24 mEq/L) are expected with compensatory low arterial partial pressure of carbon dioxide (PaCO₂) (compensatory respiratory alkalosis).

A 34-Year-Old Woman Comes to the Office Due to a 3-Day