✅ Model Answer:
• Diagnosis: Gitelman syndrome (SLC12A3 mutation – thiazide-sensitive NCC).
• Evidence: Hypokalemic metabolic alkalosis, hypomagnesemia (Mg 1.2), hypocalciuria (Ca:Cr 0.05), normal BP, fatigue, muscle cramps, carpopedal spasm.
• Next step: Genetic testing (SLC12A3). Start oral potassium chloride (2-4 mEq/kg/day) and magnesium oxide/citrate (5-10 mg/kg/day of elemental Mg). Avoid thiazide diuretics. Monitor electrolytes and symptoms.

✅ Model Answer:
• Gene: SLC12A3 located on chromosome 16q13.
• Protein: NCC (Na-Cl cotransporter) in the distal convoluted tubule.
• Inheritance: Autosomal recessive.
• Prevalence: 1 in 40,000 (more common in East Asians).
• Pathophysiology: Loss-of-function mutations in SLC12A3 → impaired NaCl reabsorption in the DCT → salt wasting, volume contraction, secondary hyperaldosteronism → hypokalemic metabolic alkalosis. Reduced Ca reabsorption (due to loss of NCC → decreased Ca entry into cells) → hypocalciuria. Mg wasting (due to TRPM6 dysfunction) → hypomagnesemia.
• Most common mutation: Various (missense, nonsense, frameshift, splice-site).

✅ Model Answer:
• Classic features:
- Hypokalemia: Muscle weakness, cramps, fatigue, paralysis (rare).
- Hypomagnesemia: Carpopedal spasm, tetany, Chvostek/Trousseau sign, muscle twitching, seizures.
- Salt craving.
- Polyuria, polydipsia (less prominent than Bartter).
- Normal blood pressure.
- Growth: Usually normal (milder than Bartter).
- Chondrocalcinosis (in adults, due to hypomagnesemia).
• Age of onset: Often adolescence or adulthood (but can present in childhood).
• Other: Prolonged QT interval (hypokalemia, hypomagnesemia), paresthesias.

✅ Model Answer:
• Biochemical:
- Serum K: Low (<3.0 mEq/L).
- Serum Mg: Low (<1.5 mEq/L).
- Serum Ca: Normal.
- Serum Na: Normal or low.
- HCO3: Elevated (metabolic alkalosis).
- Urine Ca:Cr: Low (<0.05) – hypocalciuria.
- Urine Mg: Elevated (Mg wasting).
- Plasma renin and aldosterone: Elevated (secondary hyperaldosteronism).
• Imaging: Renal ultrasound – usually normal (no nephrocalcinosis).
• Genetic testing: SLC12A3 gene sequencing (confirmatory).
• Distinguish from Bartter: Hypomagnesemia and hypocalciuria are key differentiators.

✅ Model Answer:
• Electrolyte replacement:
- Oral potassium chloride: 2-4 mEq/kg/day (up to 200 mEq/day in severe cases).
- Oral magnesium oxide/citrate: 5-10 mg/kg/day of elemental Mg (e.g., 200-400 mg/day).
- Salt supplementation: May be needed in severe salt wasting.
• Potassium-sparing diuretics: Spironolactone or amiloride – may reduce potassium loss, but caution in hypovolemia.
• Avoid: Thiazide diuretics (worsen electrolyte losses).
• Avoid: NSAIDs (may worsen renal function).
• Monitor: Serum K, Mg, BP, and symptoms.
• Education: Avoid diuretic use, maintain high salt intake, recognize symptoms of hypokalemia/hypomagnesemia.
• Genetic counseling.

✅ Model Answer:
• Hypokalemia: Muscle weakness, cramps, paralysis, arrhythmias (prolonged QT), rhabdomyolysis.
• Hypomagnesemia: Tetany, carpopedal spasm, seizures, Chvostek/Trousseau sign.
• Growth failure: Usually mild (unlike Bartter).
• Chondrocalcinosis: Calcium pyrophosphate deposition in joints (in adults).
• Chronic kidney disease: Rare; may develop with long-term hypokalemia.
• Psychosocial: Fatigue, weakness affecting daily activities.
• Arrhythmias: Due to hypokalemia and hypomagnesemia (QT prolongation).
• Recurrent urinary tract infections: Due to hypokalemia-induced nephropathy.

✅ Model Answer:
• Prognosis:
- Excellent with electrolyte replacement.
- Quality of life: Good with treatment.
- Life expectancy: Normal.
- Growth and development: Usually normal.
- Kidney function: Generally preserved.
- Fertility: Normal.
• Long-term follow-up:
- Monitor serum K, Mg, BP.
- Monitor for complications.
- Avoid thiazide diuretics.
- Genetic counseling.

✅ Model Answer:
• Gitelman syndrome:
- Site of defect: Distal convoluted tubule (NCC – thiazide-sensitive cotransporter).
- Serum Mg: Low (hypomagnesemia).
- Urine Ca: Low (hypocalciuria, Ca:Cr <0.05).
- Nephrocalcinosis: Absent.
- Age of onset: Later childhood/adolescence.
- Polyuria/polydipsia: Less prominent.
- Growth failure: Mild or absent.
- Treatment: K, Mg supplementation. Avoid thiazides.
• Bartter syndrome:
- Site of defect: Thick ascending limb of Henle (NKCC2, ROMK, ClC-Kb, barttin).
- Serum Mg: Normal.
- Urine Ca: High (hypercalciuria, Ca:Cr >0.2).
- Nephrocalcinosis: Common.
- Age of onset: Antenatal or early childhood.
- Polyuria/polydipsia: Prominent.
- Growth failure: Severe.
- Treatment: K, Mg (if needed), indomethacin.