A 5-week-old male infant is brought to the pediatric evaluation center by his mother due t… — USMLE Step 2 CK

A $5$ -week-old male infant is brought to the pediatric evaluation center by his mother due to a $5$ -day history of progressively forceful vomiting. The mother reports that the infant vomits large amounts of milk immediately after feeding but remains eager to eat again shortly afterward. The vomitus does not contain bile or blood. The infant has had fewer wet diapers over the last $2$ days. Vital signs are temperature $37.0^{\circ}\text{C}$ ( $98.6^{\circ}\text{F}$ ), heart rate $178/\text{min}$ , respiratory rate $42/\text{min}$ , and blood pressure $70/40\text{ mm Hg}$ . On physical examination, the infant appears lethargic with a sunken anterior fontanelle, dry mucous membranes, and delayed capillary refill. Palpation of the abdomen reveals a $2$ -cm, firm, mobile mass in the right upper quadrant. Laboratory studies demonstrate:

Sodium: $131\text{ mEq/L}$
Potassium: $2.7\text{ mEq/L}$
Chloride: $85\text{ mEq/L}$
Bicarbonate: $36\text{ mEq/L}$
Blood urea nitrogen: $32\text{ mg/dL}$
Creatinine: $0.9\text{ mg/dL}$

Based on the patient's presentation and laboratory findings, which of the following is the most appropriate initial step in management?

A
Immediate surgical pyloromyotomy
B
Abdominal ultrasonography
Intravenous fluid resuscitation with normal salineAnswer
D
Initiation of an extensively hydrolyzed formula
E
Intravenous administration of broad-spectrum antibiotics

Answer

Intravenous fluid resuscitation with normal saline

This patient presents with a classic history and physical examination findings of infantile hypertrophic pyloric stenosis, complicated by severe dehydration and a hypokalemic, hypochloremic metabolic alkalosis. The most critical initial step in management is to restore intravascular volume and correct the electrolyte and acid-base derangements using intravenous normal saline. Pyloric stenosis is considered a medical emergency rather than a surgical emergency. Correcting the alkalosis is paramount before any surgical intervention (pyloromyotomy) because general anesthesia in a patient with severe metabolic alkalosis significantly increases the risk of postoperative apnea, which occurs due to a depressed respiratory drive from compensatory carbon dioxide retention.

Step-by-Step Solution

Evaluate the clinical presentation and physical examination.

The infant exhibits projectile nonbilious vomiting, dehydration, and a palpable right upper quadrant 'olive' mass, which is classic for infantile hypertrophic pyloric stenosis.

To establish the most likely diagnosis before interpreting laboratory results.

Analyze the patient's hemodynamic status and laboratory findings.

The patient has severe dehydration (tachycardia, elevated BUN, sunken fontanelle) and hypokalemic, hypochloremic metabolic alkalosis.

To determine the severity of the secondary complications resulting from the gastric outlet obstruction.

Determine the most appropriate initial management step.

Immediate intravenous fluid resuscitation with normal saline is required before any surgical or diagnostic procedures.

Pyloric stenosis is a medical emergency; operating on an alkalotic infant increases the risk of postoperative respiratory failure.

Key Concept

In patients with infantile hypertrophic pyloric stenosis, the initial and most critical step in management is the correction of dehydration and hypokalemic, hypochloremic metabolic alkalosis with intravenous normal saline, prior to any surgical intervention.

Estimated Time:1m 15s

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