A 5-week-old male infant is evaluated in the emergency department for persistent vomiting … — USMLE Step 2 CK

A 5-week-old male infant is evaluated in the emergency department for persistent vomiting over the past 5 days. His parents report that he forcefully vomits large amounts of milk immediately after every feed. The vomitus does not contain bile or blood. Despite the frequent vomiting, the infant appears extremely hungry and eagerly feeds when offered. He has had only one wet diaper in the last 24 hours. He was born at full term via uncomplicated vaginal delivery. Vital signs show a heart rate of $168 \text{ /min}$ , respiratory rate of $30 \text{ /min}$ , and blood pressure of $72/40 \text{ mm Hg}$ . Physical examination reveals a lethargic infant with sunken eyes, dry mucous membranes, and a depressed anterior fontanelle. Palpation of the abdomen reveals a firm, mobile, $2 \text{-cm}$ mass in the right upper quadrant.

Laboratory studies show:
Sodium: $131 \text{ mEq/L}$
Potassium: $2.8 \text{ mEq/L}$
Chloride: $85 \text{ mEq/L}$
Bicarbonate: $35 \text{ mEq/L}$
Blood urea nitrogen (BUN): $34 \text{ mg/dL}$
Creatinine: $0.8 \text{ mg/dL}$

Which of the following is the most appropriate initial step in the management of this patient?

Administration of intravenous 0.9% sodium chlorideAnswer
B
Immediate surgical pyloromyotomy
C
Abdominal ultrasound
D
Administration of intravenous broad-spectrum antibiotics
E
Transition to an extensively hydrolyzed formula

Answer

Administration of intravenous 0.9% sodium chloride

The correct intervention is the administration of intravenous 0.9% sodium chloride. This patient presents with classic signs of hypertrophic pyloric stenosis (HPS), including projectile nonbilious vomiting, a palpable olive-shaped mass, and severe dehydration. The loss of gastric hydrochloric acid (HCl) results in a hypochloremic metabolic alkalosis. Concurrent volume depletion triggers aldosterone release, which attempts to retain sodium at the expense of excreting potassium and hydrogen ions in the distal tubule, exacerbating the hypokalemia and alkalosis. HPS is a medical emergency, not a surgical one. The absolute most important initial step in management is restoring intravascular volume and correcting the electrolyte and acid-base disturbances with isotonic saline boluses. Surgery (Ramstedt pyloromyotomy) must be delayed until the metabolic alkalosis is resolved to avoid the severe complication of postoperative apnea.

Step-by-Step Solution

Identify the primary clinical diagnosis based on the physical exam and history.

The infant has hypertrophic pyloric stenosis, indicated by the timeline (5 weeks old), nonbilious projectile vomiting, ravenous hunger, and a palpable 'olive-shaped' mass in the right upper quadrant.

Recognizing the classic clinical presentation establishes the underlying etiology of the patient's symptoms.

Analyze the laboratory findings and vital signs to determine the patient's physiologic state.

The patient is in hypovolemic shock (tachycardia, hypotension, elevated BUN/Creatinine) with severe hypochloremic, hypokalemic metabolic alkalosis (pH elevation implied by high bicarbonate).

Loss of gastric hydrochloric acid (HCl) causes the alkalosis, while volume depletion activates the renin-angiotensin-aldosterone system, leading to renal potassium wasting.

Determine the priority of interventions for hypertrophic pyloric stenosis with severe metabolic derangement.

The absolute first step is intravenous fluid resuscitation with an isotonic crystalloid (0.9% normal saline), followed by potassium supplementation once urine output is established.

Pyloric stenosis is a medical emergency, not a surgical one. Performing surgery under general anesthesia while the patient has uncorrected metabolic alkalosis significantly increases the risk of life-threatening postoperative apnea.

Key Concept

In patients with hypertrophic pyloric stenosis, severe dehydration and hypochloremic metabolic alkalosis must be medically corrected with intravenous fluids before proceeding to definitive surgical management.

Estimated Time:1m 0s

Question

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Topics

More questions from this topic