During a routine follow-up visit, the parents of a 40-day-old infant report an alarming pa… — USMLE Step 2 CK

During a routine follow-up visit, the parents of a 40-day-old infant report an alarming pattern of vomiting that began one week ago. They describe the vomiting as projectile, occurring 10 to 15 minutes after breastfeeding. The vomitus consists solely of undigested milk. Despite the frequent vomiting, the infant exhibits a strong appetite and eagerly attempts to feed again. Upon review of systems, the parents note the infant has had only one wet diaper in the past 24 hours. Vital signs in the clinic show a heart rate of 180/min, a respiratory rate of 32/min, and a blood pressure of 68/38 mm Hg. Physical examination reveals lethargy, a sunken anterior fontanelle, dry oral mucosa, and poor skin turgor. A firm, movable, 2-cm mass is palpated in the right epigastric region. Stat laboratory testing demonstrates the following:

Laboratory Parameter	Value	Reference Range
Serum sodium	$131 \text{ mEq/L}$	$135-145 \text{ mEq/L}$
Serum potassium	$2.8 \text{ mEq/L}$	$3.5-5.0 \text{ mEq/L}$
Serum chloride	$86 \text{ mEq/L}$	$98-106 \text{ mEq/L}$
Serum bicarbonate	$35 \text{ mEq/L}$	$22-28 \text{ mEq/L}$
Blood urea nitrogen	$32 \text{ mg/dL}$	$7-20 \text{ mg/dL}$
Serum creatinine	$0.9 \text{ mg/dL}$	$0.2-0.4 \text{ mg/dL}$

What is the most appropriate initial step in managing this patient's condition?

A
Urgent surgical consultation for an extramucosal pyloromyotomy
B
Referral for an urgent sonographic evaluation of the abdomen
Initiation of intravenous volume expansion with isotonic salineCevap
D
Pharmacologic intervention with a carbonic anhydrase inhibitor
E
Transition from breastfeeding to an extensively hydrolyzed formula

Cevap

Initiation of intravenous volume expansion with isotonic saline is the most critical initial step to stabilize the infant's hemodynamics and correct the hypochloremic, hypokalemic metabolic alkalosis before any surgical intervention.

The correct answer is initiation of intravenous volume expansion with isotonic saline. This patient's presentation of projectile, nonbilious vomiting, a palpable epigastric mass, and hypochloremic hypokalemic metabolic alkalosis is diagnostic of infantile hypertrophic pyloric stenosis. The patient is in hypovolemic shock (tachycardia, hypotension, severe dehydration). The absolute most critical initial step is fluid and electrolyte resuscitation with intravenous normal saline. Definitive surgical treatment (pyloromyotomy) must never be performed until the alkalosis and dehydration are fully corrected, as general anesthesia in an unresuscitated infant significantly increases the risk of postoperative apnea and mortality.

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Analyze the clinical presentation.

A 40-day-old infant presents with projectile, nonbilious vomiting, dehydration, and a palpable right epigastric mass.

These are the hallmark clinical signs of infantile hypertrophic pyloric stenosis (IHPS), making the diagnosis highly certain.

Evaluate the patient's hemodynamic status and laboratory findings.

The patient is in hypovolemic shock (HR 180, BP 68/38, lethargy) with severe hypochloremic, hypokalemic metabolic alkalosis.

Loss of gastric hydrochloric acid leads to direct chloride depletion and primary alkalosis, while volume contraction leads to secondary hyperaldosteronism and subsequent potassium wasting.

Determine the priority of interventions based on acuity.

Immediate intravenous fluid resuscitation with normal saline must be initiated.

While a pyloromyotomy is the definitive cure, anesthesia is strictly contraindicated in unresuscitated infants with metabolic alkalosis due to an exceptionally high risk of postoperative apnea. Medical stabilization is the absolute priority.

Anahtar Kavram

In infantile hypertrophic pyloric stenosis, the immediate priority is medical stabilization with intravenous fluids to correct hypovolemia and hypochloremic, hypokalemic metabolic alkalosis prior to performing definitive surgery.

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