A 4-week-old boy is brought to your OPD by his mother because he has had increasing amounts of vomiting over the past week. Initially it staed as “spitting up” after a few meals a day, but now the baby is having projectile vomiting after every meal. She says the vomitus is non-bloody and non- bilious and the baby appears hungry after he vomits. This is her first child and she is not sure if this is normal. Physical examination is unremarkable. Laboratory studies show: Sodium: 140 mEq/L Potassium: 3.0 mEq/L Chloride: 87 mEq/L Bicarbonate: 30 mEq/L At this time the most appropriate next step is to

## **Core Concept**
The clinical presentation of projectile vomiting in a 4-week-old infant, characterized by non-bloody and non-bilious vomit, suggests a diagnosis of **pyloric stenosis**. This condition is caused by hypertrophy of the pyloric muscle, leading to gastric outlet obstruction. The laboratory findings of hypokalemia (low potassium), hypochloremia (low chloride), and metabolic alkalosis (elevated bicarbonate) are classic for this condition.

## **Why the Correct Answer is Right**
The symptoms and laboratory findings in this case are typical for pyloric stenosis. The projectile vomiting leads to loss of gastric contents, which are rich in hydrogen ions (acid) and chloride. This loss results in a decrease in chloride levels (hypochloremia) and an increase in bicarbonate levels (metabolic alkalosis). The repeated vomiting also leads to loss of potassium, causing hypokalemia. The appropriate next step in managing a patient with suspected pyloric stenosis is to perform an **ultrasound of the abdomen**. The ultrasound is highly sensitive and specific for diagnosing pyloric stenosis by demonstrating the thickened pyloric muscle and the elongated pyloric canal.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While a complete blood count (CBC) and blood cultures might be part of a general workup for a vomiting infant, they are not specific or diagnostic for pyloric stenosis.
- **Option B:** An upper GI series (barium study) can be used to diagnose pyloric stenosis but is not the first line due to the risk of aspiration and the availability of safer, more accurate methods like ultrasound.
- **Option C:** Administering intravenous fluids and correcting electrolytes is crucial in the management of pyloric stenosis but does not represent the next step in diagnosis.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is the classic description of pyloric stenosis as " projectile vomiting that is non-bilious and occurs in a hungry infant," often summarized by the mnemonic **"projectile, non-bilious, hungry, and thriving"**. The ultrasound finding of a pyloric muscle thickness >3 mm and a pyloric canal length >15 mm in infants is diagnostic.

## **Correct Answer:** . Ultrasound of the abdomen.

✓ Correct Answer: C. obtain an abdominal ultrasound