A 1-month-old boy is evaluated in the pediatric emergency department due to a 4-day histor… — USMLE Step 2 CK

A 1-month-old boy is evaluated in the pediatric emergency department due to a 4-day history of progressively forceful vomiting. The parents report that he eagerly feeds but regurgitates the entire volume immediately afterward. The vomitus consists of undigested milk and does not appear green. The infant has not had a bowel movement in two days and is producing fewer wet diapers. Vital signs show a heart rate of $170/\text{min}$ , respiratory rate of $35/\text{min}$ , and blood pressure of $75/45 \text{ mm Hg}$ . Physical examination reveals a lethargic infant with a sunken anterior fontanelle, dry mucous membranes, and delayed capillary refill. Palpation of the abdomen reveals a $2\text{-cm}$ , firm, mobile mass in the right epigastrium. Laboratory studies reveal a pH of $7.52$ , serum sodium of $132 \text{ mEq/L}$ , potassium of $2.9 \text{ mEq/L}$ , and chloride of $88 \text{ mEq/L}$ . What is the most appropriate next step in the management of this patient?

A
Urgent surgical pyloromyotomy
B
Abdominal ultrasonography
Intravenous administration of 0.9% normal salineAnswer
D
Switch to a hydrolyzed infant formula
E
Intravenous administration of broad-spectrum antibiotics

Answer

Intravenous administration of 0.9% normal saline

The correct response is intravenous administration of 0.9% normal saline. This infant presents with the classic triad of hypertrophic pyloric stenosis (HPS): projectile non-bilious vomiting, an 'olive-shaped' abdominal mass, and hypochloremic, hypokalemic metabolic alkalosis. The prolonged vomiting of gastric hydrochloric acid leads to loss of fluid, hydrogen ions, and chloride. The kidneys attempt to conserve volume by activating the renin-angiotensin-aldosterone system, leading to potassium depletion. Because this patient has signs of severe dehydration and hypovolemic shock (lethargy, tachycardia, hypotension, poor capillary refill), the absolute most critical next step is medical resuscitation. Intravenous fluid therapy with normal saline must be initiated immediately to restore intravascular volume, correct the alkalosis, and replenish electrolytes. Pyloric stenosis is a medical emergency, not an immediate surgical emergency.

Step-by-Step Solution

Analyze the clinical presentation and laboratory values.

Identify hypertrophic pyloric stenosis presenting with severe dehydration and hypochloremic, hypokalemic metabolic alkalosis.

Projectile non-bilious vomiting, an 'olive-shaped' epigastric mass, and specific acid-base derangements are classic for this condition.

Determine the immediate clinical priority.

Prioritize fluid and electrolyte resuscitation over diagnostic imaging or definitive surgical intervention.

The patient is hemodynamically compromised (tachycardia, hypotension, lethargy); inducing anesthesia for surgery in this state carries a high risk of life-threatening complications.

Key Concept

Initial management of hypertrophic pyloric stenosis requires prompt correction of dehydration and hypochloremic, hypokalemic metabolic alkalosis with intravenous normal saline before pursuing imaging or surgery.

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