✅ Model Answer:
• Diagnosis: Nephrogenic Diabetes Insipidus (ADH resistance) — hypernatremia (152 mEq/L), elevated serum osmolality (310 mOsm/kg), dilute urine (150 mOsm/kg), normal glucose. Water deprivation test shows no concentration (urine osm remains 160). DDAVP (desmopressin) shows <10% rise in urine osmolality (from 160 to 190), confirming nephrogenic DI. Family history of polyuria in maternal uncle suggests X-linked inheritance (AVPR2 mutation).
• Any other test: Genetic testing (AVPR2 for X-linked, AQP2 for autosomal). Serum calcium, potassium, lithium level (if exposure). Renal ultrasound (obstructive uropathy, nephrocalcinosis). Copeptin level (elevated in nephrogenic DI).
• What to do next: Thiazide diuretic (hydrochlorothiazide 1-2 mg/kg/day) + amiloride (if potassium-wasting) or indomethacin (2 mg/kg/day) to reduce urine output. Low-sodium diet. Ensure adequate free water access.
• Follow-up plan: Monitor serum sodium, potassium, growth. Ensure adequate free water access. Genetic counseling for family. Avoid medications that worsen (lithium). Regular nephrology/endocrinology follow-up.

✅ Model Answer:
• Nephrogenic Diabetes Insipidus (NDI) is a disorder characterized by resistance of the kidney to antidiuretic hormone (ADH/vasopressin), leading to polyuria, polydipsia, and dilute urine despite elevated ADH.
• Causes:
- Genetic (congenital):
• X-linked: AVPR2 gene mutation (most common — 90% of congenital NDI).
• Autosomal recessive: AQP2 gene mutation (water channel).
• Autosomal dominant: AQP2 gene mutation (rare).
- Acquired:
• Drugs: Lithium (most common), amphotericin B, ifosfamide, foscarnet, demeclocycline.
• Metabolic: Hypercalcemia, hypokalemia.
• Renal: Sickle cell nephropathy, chronic kidney disease, obstructive uropathy, pyelonephritis.
• Other: Polycystic kidney disease, medullary sponge kidney, sarcoidosis.
- Family history: X-linked inheritance (maternal uncle affected, as in this case).

✅ Model Answer:
• Key differences:
- Central DI:
• Urine osmolality remains <300 during water deprivation.
• After DDAVP: >50% rise in urine osmolality (or >600 mOsm/kg).
• Mechanism: ADH deficiency.
• Treatment: DDAVP (desmopressin).
- Nephrogenic DI:
• Urine osmolality remains <300 during water deprivation.
• After DDAVP: <50% rise in urine osmolality (or <600 mOsm/kg).
• Mechanism: ADH resistance.
• Treatment: Thiazide diuretics + amiloride, low-sodium diet.
- Primary polydipsia:
• Urine osmolality rises to >800 during water deprivation (normal ADH response).
• After DDAVP: No further rise (urine is already concentrated).
• Mechanism: Excessive water intake suppresses ADH.
• Treatment: Fluid restriction, behavior therapy.

✅ Model Answer:
• Genetic testing: Essential for confirming the diagnosis and guiding management in congenital NDI.
• Genes:
- AVPR2: X-linked (90% of congenital NDI). Located on chromosome Xq28.
- AQP2: Autosomal recessive (10%) or autosomal dominant (rare). Located on chromosome 12q13.
• Indications:
- Confirming the diagnosis in a child with NDI.
- Prenatal diagnosis in families with known mutations.
- Carrier testing in females (X-linked).
- Genetic counseling for family planning.
• Clinical implications:
- X-linked: Males are affected; females may have variable symptoms (due to X-inactivation).
- Autosomal recessive: Both parents are carriers (25% recurrence risk).
- Genetic testing can guide prognosis and management.

✅ Model Answer:
• Thiazide diuretics:
- Hydrochlorothiazide: 1-2 mg/kg/day (paradoxically reduces urine output by inducing mild hypovolemia).
- Amiloride: Added if potassium-wasting is a concern (especially if lithium-induced).
• Indomethacin:
- 2 mg/kg/day (inhibits prostaglandins, enhances the effect of thiazides).
- Used in severe cases (monitor renal function).
• Low-sodium diet: Reduces urine output.
• Ensure adequate free water: Access to water ad libitum (prevent hypernatremia).
• Avoid:
- Lithium, amphotericin B, other nephrotoxic drugs.
- Dehydration (especially during illness).
• Monitor: Serum sodium, potassium, urine output, growth.

✅ Model Answer:
• Complications:
- Hypernatremia: Severe dehydration, hyperosmolar state → seizures, coma, death.
- Failure to thrive: Poor weight gain, growth failure (due to chronic dehydration and poor appetite).
- Enuresis/nocturia: Sleep disturbance, fatigue.
- Chronic kidney disease: If prolonged or severe (especially if associated with obstruction or nephrocalcinosis).
- Nephrocalcinosis: Due to hypercalciuria (may occur with thiazide use if not monitored).
- Psychosocial: Impact on school, social activities (frequent trips to the bathroom).
- Electrolyte imbalances: Hypokalemia (with thiazide therapy), hypercalcemia (if underlying cause).
- Hydrocephalus: Rare, but can occur in infants with severe dehydration.

✅ Model Answer:
• Renal ultrasound: Important for evaluating structural causes of NDI.
• Findings:
- Obstructive uropathy: Hydronephrosis, hydroureter (posterior urethral valves, stricture).
- Nephrocalcinosis: Hyperechogenicity of the medullary pyramids (may be associated with hypercalcemia).
- Medullary sponge kidney: Dilated collecting ducts (rare).
- Polycystic kidney disease: Cysts (if associated).
- Small/echogenic kidneys: Chronic kidney disease.
• Indications:
- All children with NDI should have a renal ultrasound to rule out structural abnormalities.
- If nephrocalcinosis is found, check calcium, phosphate, PTH.
- If obstructive uropathy is found, refer to urology.
• Follow-up: Repeat ultrasound if symptoms worsen or renal function declines.

✅ Model Answer:
• Prognosis:
- Variable: Depends on the underlying cause (congenital vs acquired).
- Congenital (X-linked): Lifelong condition, but can be managed with thiazides and low-sodium diet.
- Acquired: May resolve if the underlying cause is treated (e.g., stopping lithium, correcting hypercalcemia).
- Growth: May improve with adequate hydration and nutrition.
- Renal function: Usually preserved if managed well; however, chronic dehydration can lead to CKD.
- Life expectancy: Normal if treated and monitored appropriately.
• Long-term follow-up:
- Monitor: Serum sodium, potassium, urine output, growth, blood pressure.
- Renal function: Monitor creatinine, BUN annually.
- Genetic counseling: For family members (X-linked inheritance).
- Education: Teach child and family about the importance of free water access, signs of dehydration, and sick day rules.
- Psychosocial support: Address school and social challenges.