USMLE Step 1 Acid–Base Disorders Practice Questions
Acid-base disorders are common clinical problems that require a solid understanding of renal and respiratory physiology. For USMLE Step 1, focus on identifying the primary disorder, calculating compensation, and understanding the underlying mechanisms and causes of metabolic and respiratory acid-base imbalances.
Patient Information Age: 68 years | Gender: F, self-identified | Race/Ethnicity: Caucasian, self-identified | Site of Care: Emergency Department History Reason for Visit/Chief Concern: "I've been short of breath for a few days." History of Present Illness: • Progressive shortness of breath, lethargy, and a productive cough with yellow sputum for 3 days. • History of chronic obstructive pulmonary disease (COPD) and hypertension. Physical Examination Temp: 37.2°C | Pulse: 98/min | Resp: 30/min | BP: 140/85 mm Hg • Appears distressed, using accessory muscles to breathe. • Bilateral expiratory wheezes and prolonged expiration. Diagnostic Studies Arterial Blood Gas (ABG): pH: 7.25 PCO2: 60 mm Hg Bicarbonate (HCO3-): 28 mEq/L PO2: 65 mm Hg Which of the following best describes this patient's primary acid-base disorder?
A 45-year-old man with a history of type 1 diabetes mellitus presents to the emergency department with 1 day of nausea, vomiting, and abdominal pain. He reports poor adherence to his insulin regimen. Physical examination reveals Kussmaul respirations and a fruity odor on his breath. Laboratory studies show: pH 7.18, PCO2 25 mm Hg, HCO3- 10 mEq/L, Na+ 135 mEq/L, K+ 4.5 mEq/L, Cl- 95 mEq/L. Which of the following is the most likely cause of this patient's acid-base disturbance?
A 28-year-old woman is admitted to the hospital for severe anxiety and hyperventilation following a panic attack. Her arterial blood gas (ABG) results are pH 7.55, PCO2 28 mm Hg, and HCO3- 22 mEq/L. Which of the following is the expected renal compensatory response to this acid-base disturbance?
A 58-year-old man with a history of chronic kidney disease (CKD) presents for a routine follow-up. He reports feeling generally well but sometimes experiences fatigue. His current medications include lisinopril and furosemide. Laboratory results show: pH 7.32, PCO2 35 mm Hg, HCO3- 18 mEq/L, Na+ 138 mEq/L, K+ 5.2 mEq/L, Cl- 105 mEq/L, Creatinine 3.5 mg/dL. Which of the following mechanisms primarily contributes to this patient's acid-base disorder?
A 72-year-old woman with a history of severe congestive heart failure and chronic diuretic use (hydrochlorothiazide) presents with muscle weakness and fatigue. Her arterial blood gas (ABG) shows: pH 7.50, PCO2 48 mm Hg, HCO3- 35 mEq/L. Her serum potassium is 2.8 mEq/L. Which of the following is the most likely underlying cause of her acid-base imbalance?
Acid–Base Disorders — frequently asked
What is the most important first step when interpreting ABG results for USMLE Step 1?
The most important first step is to assess the pH to determine if the primary disorder is acidosis or alkalosis. Then, look at PCO2 and HCO3- to identify the primary respiratory or metabolic component.
How do I calculate the anion gap, and why is it important?
The anion gap is calculated as Na+ - (Cl- + HCO3-). A normal range is typically 8-12 mEq/L. It's crucial for differentiating causes of metabolic acidosis into anion gap (e.g., DKA, lactic acidosis) and non-anion gap (e.g., renal tubular acidosis, diarrhea) categories.
What are the expected compensatory responses for each primary acid-base disorder?
For metabolic acidosis, the body compensates with respiratory alkalosis (hyperventilation, decreased PCO2). For metabolic alkalosis, it's respiratory acidosis (hypoventilation, increased PCO2). For respiratory acidosis, it's metabolic alkalosis (renal HCO3- retention). For respiratory alkalosis, it's metabolic acidosis (renal HCO3- excretion). The compensation formulas (e.g., Winter's formula) are important to know for assessing mixed disorders.
Should I memorize all the specific causes for each acid-base disorder?
While it's helpful to know common causes, focus more on understanding the underlying pathophysiological mechanisms (e.g., why DKA causes an anion gap acidosis, why diuretics cause metabolic alkalosis). This mechanistic understanding will allow you to deduce causes even if you haven't memorized every single one.
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