✅ Model Answer:
• Diagnosis: Bartter syndrome type IV (BSND mutation – barttin deficiency).
• Evidence: Hypokalemic hypochloremic metabolic alkalosis, normal BP, hypercalciuria (Ca:Cr 0.4), sensorineural deafness, failure to thrive, recurrent dehydration.
• Next step: Genetic testing (BSND). Start potassium chloride, magnesium supplementation, indomethacin (1-2 mg/kg/day), and hearing aids/cochlear implants. Monitor electrolytes, growth, and renal function.

✅ Model Answer:
• Gene: BSND (barttin) located on chromosome 1p32.3.
• Protein: Barttin is the β-subunit for the chloride channels ClC-Ka and ClC-Kb in the kidney (thick ascending limb) and inner ear (stria vascularis).
• Inheritance: Autosomal recessive.
• Pathophysiology: Mutations in BSND impair ClC-Ka/ClC-Kb function → impaired chloride reabsorption in the thick ascending limb → salt wasting, hypokalemic alkalosis, hypercalciuria, nephrocalcinosis. In the inner ear, defective ClC-K channels cause endolymph production defects → sensorineural deafness.
• Prevalence: Rare; accounts for ~10% of Bartter syndrome cases.
• Deafness: Present in all patients with BSND mutations (type IV).

✅ Model Answer:
• Classic features:
- Sensorineural deafness (congenital or early-onset – hallmark of type IV).
- Hypokalemic metabolic alkalosis (K <3.0, HCO3 >30).
- Hypercalciuria (Ca:Cr >0.2) → nephrocalcinosis, renal stones.
- Salt wasting → recurrent dehydration, failure to thrive.
- Polyuria, polydipsia.
- Normal blood pressure.
- Growth failure.
• Other: Muscle cramps, weakness, fatigue.
• Age of onset: Usually in infancy or early childhood.

✅ Model Answer:
• Biochemical:
- Serum K: Low.
- HCO3: Elevated (metabolic alkalosis).
- Serum Na: Normal or low.
- Serum Cl: Low (hypochloremia).
- Serum Ca: Normal.
- Urine Ca:Cr: >0.2 (hypercalciuria).
- Plasma renin and aldosterone: Elevated (secondary hyperaldosteronism).
• Audiometry: Sensorineural hearing loss.
• Renal ultrasound: Nephrocalcinosis (medullary), renal stones.
• Genetic testing: BSND gene sequencing (confirmatory).
• Distinguish from other Bartter types: Deafness is specific to type IV.

✅ Model Answer:
• Electrolyte replacement:
- Oral potassium chloride: 4-6 mEq/kg/day (higher doses may be needed).
- Magnesium supplementation: If hypomagnesemia is present.
- Sodium chloride: May be needed in severe salt wasting.
• Prostaglandin synthetase inhibitors (indomethacin):
- Dose: 1-2 mg/kg/day (divided q12h).
- Mechanism: Reduces prostaglandin E2-mediated salt wasting and improves growth.
- Monitor: Renal function, GI side effects.
• Hearing aids or cochlear implants: For sensorineural deafness.
• Nutritional support: Calorie supplementation.
• Avoid: NSAIDs in patients with renal impairment.
• Monitor: Electrolytes, growth, renal function, and hearing.

✅ Model Answer:
• Sensorineural deafness: May be profound; hearing aids/cochlear implants are essential.
• Growth failure: Poor weight gain, short stature.
• Nephrocalcinosis: Calcium deposition in the renal medulla → can lead to CKD.
• Nephrolithiasis: Renal stones (calcium oxalate/phosphate).
• Chronic kidney disease (CKD): Progressive renal impairment.
• Hypokalemia: Muscle weakness, cramps, arrhythmias.
• Dehydration: Recurrent, especially in infancy.
• Psychosocial: Impact of hearing loss and chronic illness.

✅ Model Answer:
• Prognosis:
- Variable: Depends on the severity of renal and hearing impairment.
- Renal function: May decline over time (nephrocalcinosis, NSAID nephrotoxicity).
- Growth: Can improve with indomethacin and electrolyte replacement.
- Hearing: Early intervention with hearing aids/cochlear implants improves quality of life.
- Quality of life: Good with multidisciplinary care.
• Long-term follow-up:
- Monitor electrolytes, BP, renal function.
- Monitor growth and nutrition.
- Monitor for nephrocalcinosis (renal ultrasound).
- Audiometry and hearing support.
- Genetic counseling.

✅ Model Answer:
• Type IV (BSND):
- Gene: BSND (barttin).
- Protein: β-subunit for ClC-Ka/Kb.
- Deafness: Sensorineural (present).
- Nephrocalcinosis: Common.
- Hypercalciuria: Yes.
- Age of onset: Infancy.
- Inheritance: Autosomal recessive.
• Type I (NKCC2): Antenatal, severe salt wasting, no deafness.
• Type II (ROMK): Antenatal, severe, no deafness.
• Type III (ClC-Kb): Classic Bartter, later onset, no deafness.
• Type V (MAGED2): X-linked, transient antenatal Bartter, no deafness.
• Key differentiator: Sensorineural deafness is unique to type IV.