Pyloric Stenosis Diagnosis

The infant's presentation of repeated non-bilious vomiting, along with an olive-shaped mass in the epigastrium, strongly suggests pyloric stenosis. This condition occurs due to hypertrophy of the pyloric sphincter muscle, causing a gastric outlet obstruction.

Vomiting and Metabolic Changes

The repeated vomiting associated with pyloric stenosis leads to significant loss of gastric contents. Key consequences include:

• Loss of Hydrochloric Acid (HCl): Gastric fluid is rich in acid ($H^+$ and $Cl^-$). Losing this fluid leads to a deficit of hydrogen ions in the body.
• Volume Depletion: Significant fluid loss occurs due to vomiting.
• Renal Compensation: The kidneys attempt to compensate for volume loss and maintain acid-base balance by retaining sodium and bicarbonate ($HCO_3^-$) and excreting hydrogen ions ($H^+$).

Expected Metabolic Abnormality

The primary metabolic consequence of losing large amounts of gastric acid ($HCl$) is metabolic alkalosis. This occurs because:

• The loss of $H^+$ directly increases the blood pH.
• The body retains bicarbonate ($HCO_3^-$) to compensate, further increasing alkalinity.
• Loss of chloride ($Cl^-$) also contributes, leading to hypochloremia, often termed hypochloremic metabolic alkalosis.
• Potassium ($K^+$) is also lost during vomiting, potentially leading to hypokalemia, but alkalosis is the direct result of acid loss.

Therefore, the most expected metabolic abnormality is metabolic alkalosis.