A 3-week-old infant presents with persistent vomiting and a palpable olive-shaped mass in the upper abdomen. Which electrolyte imbalances may be seen in order of pH, HCO₃⁻ and CO₂?

Appeared in: NORCET 8 Prelims-2025

Explanation

The clinical presentation is classic for hypertrophic pyloric stenosis (HPS).
Persistent vomiting in HPS causes a significant loss of hydrochloric acid (HCl) from the stomach.
This loss of acid (H+ and Cl-) leads to a predictable acid-base disturbance: hypochloremic metabolic alkalosis.
Metabolic alkalosis is characterized by a high pH (greater than 7.45) and a high bicarbonate (HCO3−) level (greater than 26 mEq/L).
The values of pH 7.50 and HCO3− 29 mEq/L clearly indicate metabolic alkalosis, which is the expected finding.

Option A: These values (pH 7.45, HCO3− 26 mEq/L) are at the upper limits of the normal range. While an infant might pass through this stage, the described persistent vomiting would lead to a more pronounced alkalosis.
Option C: These values (pH 7.35, HCO3− 22 mEq/L) represent a normal acid-base balance. This is inconsistent with the significant fluid and electrolyte shifts caused by persistent vomiting.
Option D: These values (pH 7.30, HCO3− 20 mEq/L) indicate metabolic acidosis. This is caused by a loss of bicarbonate or an increase in acid, not the loss of gastric acid.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Acid-Base Imbalance in Hypertrophic Pyloric Stenosis as background academic context rather than a clinical decision trigger.
Recognizing the signs of HPS and its associated metabolic alkalosis is critical for nurses, as prompt intervention is required.
Pre-operative management is focused on correcting dehydration and electrolyte imbalances with IV fluids. Surgery is not performed until the infant is rehydrated and the alkalosis is corrected to prevent post-operative complications like apnea.
Post-operative nursing care involves careful reintroduction of feedings, monitoring for continued vomiting, and assessing the surgical incision site.

How to Approach the Question

First, analyze the clinical scenario provided in the question stem. Identify the patient's age (3-week-old infant) and key symptoms (persistent vomiting, palpable olive-shaped mass).
Recognize that this combination of signs is the classic presentation for hypertrophic pyloric stenosis (HPS).
Recall the core pathophysiology of HPS: the obstruction causes forceful vomiting of stomach acid (hydrochloric acid).
Predict the resulting acid-base imbalance. The loss of acid (H+) will cause the blood to become more alkaline, leading to metabolic alkalosis.
Characterize metabolic alkalosis by its ABG values: high pH (greater than 7.45) and high bicarbonate (HCO3−) (greater than 26 mEq/L).
Evaluate the given options to find the set of values that matches the profile of metabolic alkalosis.

Concept Tested & Keywords

Concept Tested: Acid-Base Imbalance in Hypertrophic Pyloric Stenosis
Stem keywords: 3-week-old infant, persistent vomiting, palpable olive-shaped mass, electrolyte imbalances
Lead-in keywords: Which electrolyte imbalances may be seen
Clinical cues: The combination of projectile vomiting and a palpable 'olive' mass in an infant is pathognomonic for hypertrophic pyloric stenosis.
Clinical cues: Persistent vomiting of gastric contents points towards an upper GI obstruction and loss of hydrochloric acid.

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E6 Nelson Textbook of Pediatrics(2024) — Volume 2 p. 58-60

E6 Nursing Fundamentals Potter Perry 12e Part 5 p. 57-59

E6 Textbook of Biochemistry for medical StudentsDM Vasudevan Part 2 — Subpart A (pp 1-239 of 478) p. 82-84

Practise the full NORCET 8 Prelims-2025

Attempt every question from this paper in a timed mock, then review the full solution for each one.