Hypertrophic Pyloric Stenosis (HPS) — classic presentation:

• Age: 2–8 weeks (peak at 3–6 weeks)
• Gender: Males > females (4:1 ratio)
• Projectile, non-bilious vomiting (post-pyloric, so no bile)
• Hungry after vomiting (not ill-appearing)
• Olive-shaped mass palpable in right epigastrium (hypertrophied pylorus)
• Visible peristaltic waves (left to right)

Electrolyte imbalance:

Repeated vomiting of gastric acid (HCl) causes loss of H⁺ and Cl⁻:

\[\text{Loss of HCl} \Rightarrow \downarrow \text{Cl}^- \Rightarrow \uparrow \text{HCO}_3^- \Rightarrow \text{Metabolic Alkalosis}\]

Kidney compensates by retaining H⁺ and excreting K⁺ → Hypokalemia

Result: Hypochloremic, hypokalemic metabolic alkalosis

Diagnosis: Ultrasound (pyloric muscle thickness \(\geq 4\,\text{mm}\), channel length \(\geq 16\,\text{mm}\))

Treatment: First correct electrolytes and dehydration, then Ramstedt pyloromyotomy (surgical — curative).