Acid-Base Disorders USMLE Step 1 Practice Question
A 28-year-old woman with systemic lupus erythematosus presents to the clinic with a 3-week history of progressive muscle weakness, polyuria, and polydipsia. She has been on prednisone 20 mg daily for the past 6 months. Vital signs are stable (BP 118/76 mmHg, HR 88 bpm, RR 14 breaths/min, temperature 37°C). Laboratory studies show serum potassium 3.1 mEq/L (normal 3.5-5.0), serum chloride 92 mEq/L (normal 96-106), serum HCO3− 28 mEq/L (normal 22-26). Arterial blood gas: pH 7.45, PCO2 38 mmHg, HCO3− 28 mEq/L. Urinalysis shows no glycosuria, ketonuria, or proteinuria. Which of the following best explains her acid-base status?
Answer choices
- ARespiratory alkalosis with appropriate metabolic compensation
- BMetabolic acidosis with appropriate respiratory compensation
- CMetabolic alkalosis with concurrent hypokalemic hypochloremiaCorrect answer
- DMixed respiratory and metabolic acidosis
- ENormal acid-base balance with isolated hypokalemia
- FHigh anion gap metabolic acidosis
See the full explanation
Get the correct-answer rationale, why each distractor is wrong, the underlying mechanism, and high-yield associations — plus adaptive practice that targets your weak areas — with a free MedBoardPRO account.