✅ Model Answer:
• Diagnosis: Hypertrophic pyloric stenosis — projectile vomiting, palpable olive, hypochloremic metabolic alkalosis, ultrasound shows thickened pylorus (muscle ≥4mm, channel ≥15mm).
• Any other test: Ultrasound abdomen for Pyloric muscle thickness 5 mm, channel length 20 mm, Renal function tests (BUN, creatinine), pre-operative labs (CBC, coagulation profile), ECG if severe hypokalemia.
• What to do next: Correct dehydration and electrolytes (IV 0.9% saline + KCl). Surgical consult for pyloromyotomy (Ramstedt procedure).
• Follow-up plan: Post-op feeding advancement (start 4-6 hours after surgery), monitor weight gain, wound care, ensure adequate hydration.

✅ Model Answer:
• Hypochloremic metabolic alkalosis — classic electrolyte pattern:
- Low serum chloride (↓Cl⁻)
- Low serum potassium (↓K⁺)
- Elevated pH (>7.45)
- Elevated bicarbonate (↑HCO₃⁻)
- Paradoxical aciduria (kidneys excrete H⁺ to conserve K⁺)

✅ Model Answer:
• Pathophysiology: Idiopathic hypertrophy of the pyloric muscle (circular and longitudinal layers) → progressive narrowing of the pyloric canal → gastric outlet obstruction → forceful projectile vomiting.
• Loss of gastric contents (H⁺, Cl⁻, K⁺) leads to hypochloremic metabolic alkalosis.
• The exact cause is unknown but associated with genetic factors (linkage to chromosome 11q14-22) and environmental factors.

✅ Model Answer:
• Ultrasound criteria for HPS:
- Pyloric muscle thickness ≥4 mm (normal 1.5-2.5 mm)
- Pyloric channel length ≥15-20 mm (normal 10-12 mm)
- Pyloric diameter ≥14 mm
- "Target sign" or "doughnut sign" on transverse view
- "Cervix sign" on longitudinal view
- Lack of passage of gastric contents through the pylorus

✅ Model Answer:
• Initial management (pre-operative):
- IV fluid resuscitation with 0.9% saline + KCl to correct dehydration and electrolyte imbalance
- Monitor urine output and electrolytes
- Correct hypochloremia and hypokalemia before surgery
- Keep NPO (nil per os)
- NG tube decompression if severe distension
- Surgical consult for pyloromyotomy (Ramstedt procedure)

✅ Model Answer:
• Ramstedt pyloromyotomy is the definitive surgical treatment.
• Procedure: Longitudinal incision of the hypertrophied pyloric muscle down to the submucosa, allowing the mucosa to bulge through, relieving the obstruction.
• Can be performed open (small right upper quadrant incision) or laparoscopic.
• Post-operative feeding: Start with small frequent feeds (usually 4-6 hours after surgery) and advance gradually.

✅ Model Answer:
• Complications of untreated HPS:
- Severe dehydration and electrolyte imbalance
- Hypochloremic metabolic alkalosis → cardiac arrhythmias
- Failure to thrive and poor weight gain
- Malnutrition
- Aspiration pneumonia
- Jaundice (indirect hyperbilirubinemia)
- Gastric perforation (rare)
- Death (if severe dehydration and electrolyte imbalance are not corrected)

✅ Model Answer:
• Post-operative follow-up:
- Feeding advancement: Start with small frequent feeds (4-6 hours post-op), advance as tolerated
- Monitor weight gain and hydration status
- Monitor for surgical complications (bleeding, perforation, infection)
- Follow-up with paediatric surgeon
• Prognosis: Excellent with prompt diagnosis and surgical treatment. Most infants recover completely with normal growth and development. No long-term gastrointestinal sequelae.