USMLE PRACTICE QUESTION #115

TOPIC: GI Pathology

Dec 27, 2024

USMLE practice question #115 assesses our knowledge of a commonly tested newborn GI condition.

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A 3-week-old male presents to the emergency department with progressive vomiting over the past week. His parents report that the vomiting is projectile, non-bilious, and occurs after every feeding. The infant has been less active and appears dehydrated. On physical examination, the infant is lethargic with sunken eyes and dry mucous membranes. Palpation of the abdomen reveals a firm, olive-shaped mass in the epigastrium. Laboratory findings show the following:

Sodium: 132 mEq/L
Potassium: 2.8 mEq/L
Chloride: 88 mEq/L
Bicarbonate: 35 mEq/L

Which of the following acid-base disturbances is most likely in this patient?

A) High anion gap metabolic acidosis
B) Non-anion gap metabolic acidosis
C) Metabolic alkalosis with respiratory compensation
D) Mixed metabolic acidosis and respiratory alkalosis
E) Respiratory acidosis with metabolic compensation

Detailed Breakdown of Answers + Correct Answer Below ⏬

ANSWER + QUESTION BREAKDOWN

It’s important to adopt the correct MENTAL MODEL when answering USMLE questions; it saves time and increases accuracy. The mental model outlined below is a foundational component of our test-taking skills masterclass (check it out if you want to elevate your skills). Here’s how to think through this question:

Step 1. Read the last line to get to the heart of the question: “Which of the following acid-base disturbances is most likely in this patient?”

Step 2: Is this a first-, second-, or third-order question?
Answer: 2nd order. 1st: Identify the underlying problem; 2nd: Assign an appropriate acid-base disturbance to the underlying problem.

Step 3: Read the vignette carefully and ask yourself: “Based on the patient’s problem, what type of acid-base disturbance do I expect?”

Step 4. Look at the answer choices and select the option most closely resembling your final thought from “Step 3” above.

GENERAL ANALYSIS

This vignette describes a 3-week-old male infant presenting with progressive projectile, non-bilious vomiting after every feeding, along with signs of dehydration (lethargy, sunken eyes, dry mucous membranes). On physical examination, a firm, olive-shaped mass is palpated in the epigastrium, highly suggestive of pyloric stenosis.

ANSWER CHOICES:

CHOICE A: High anion gap metabolic acidosis

Explanation: High anion gap metabolic acidosis occurs when there is an accumulation of acids (e.g., lactic acidosis, diabetic ketoacidosis) or loss of bicarbonate.

CHOICE B: Non-anion gap metabolic acidosis

Explanation: Non-anion gap metabolic acidosis occurs when bicarbonate is lost from the body without an increase in unmeasured anions (e.g., diarrhea or renal tubular acidosis).

CHOICE C: Metabolic alkalosis with respiratory compensation

Explanation: Metabolic alkalosis occurs when there is a loss of hydrogen ions or a gain in bicarbonate. In this case, persistent vomiting leads to a loss of gastric acid (HCl), resulting in metabolic alkalosis. The body compensates by reducing the respiratory rate to retain CO₂ and increase carbonic acid levels.

CHOICE D: Mixed metabolic acidosis and respiratory alkalosis

Explanation: Mixed disorders involve both metabolic acidosis and respiratory alkalosis occurring simultaneously.

CHOICE E: Respiratory acidosis with metabolic compensation

Explanation: Respiratory acidosis occurs when impaired ventilation leads to CO₂ retention. Metabolic compensation involves increased bicarbonate retention by the kidneys.

FINAL VERDICT…

The most likely acid-base disturbance in this infant with pyloric stenosis is metabolic alkalosis with respiratory compensation caused by persistent vomiting, leading to loss of gastric acid. The laboratory findings—elevated bicarbonate, hypokalemia, and hypochloremia—are classic for this condition.

KEY CONCEPTS:

In pyloric stenosis, the hypertrophied pylorus obstructs gastric emptying, leading to persistent vomiting. Since the vomitus is non-bilious and consists mainly of stomach contents (which are rich in hydrochloric acid), the loss of gastric acid leads to metabolic alkalosis.
The body compensates for the loss of hydrogen ions (H⁺) by increasing bicarbonate retention, leading to elevated bicarbonate levels.
Hypokalemia and hypochloremia result from the loss of potassium and chloride in the vomitus.
The kidneys attempt to compensate for the alkalosis by retaining hydrogen ions and excreting potassium, further worsening hypokalemia.

CORRECT ANSWER: C) Metabolic alkalosis with respiratory compensation