✅ Model Answer:
• Diagnosis: Lithium-induced nephrogenic diabetes insipidus — lithium blocks ADH effect on the collecting duct. Dilute urine (120 mOsm/kg), hypernatremia (150 mEq/L), no DDAVP response (urine osm 130 → 150, <50% rise). Normal glucose excludes DM. History of lithium therapy for bipolar disorder is the key clue.
• Any other test: Serum lithium level (therapeutic or toxic range), renal function (creatinine, BUN), urine electrolytes, renal ultrasound (to assess for nephrogenic DI, structural damage, or nephrocalcinosis).
• What to do next: Reduce or stop lithium if possible (in consultation with psychiatry). Start amiloride (blocks lithium entry into collecting duct) or hydrochlorothiazide to reduce urine output. Low-sodium diet.
• Follow-up plan: Monitor renal function, serum sodium, lithium levels. Consider alternative mood stabilizers. Avoid dehydration. Regular nephrology/psychiatry follow-up.

✅ Model Answer:
• Mechanism: Lithium enters the collecting duct cells via the epithelial sodium channel (ENaC) and inhibits the action of antidiuretic hormone (ADH/vasopressin).
• Key steps:
- Lithium inhibits adenylyl cyclase → decreases cAMP production.
- Reduces the insertion of aquaporin-2 (AQP2) water channels into the apical membrane of collecting duct cells.
- Results in ADH resistance → inability to concentrate urine → polyuria and polydipsia.
• Risk factors:
- Duration of lithium therapy (>2 years increases risk).
- High lithium levels (therapeutic or toxic).
- Concurrent use of other nephrotoxic drugs (NSAIDs, ACE inhibitors).
- Pre-existing renal impairment.
• Reversibility: May be reversible if lithium is stopped early; chronic use can cause irreversible nephrogenic DI and chronic kidney disease.

✅ Model Answer:
• Amiloride: A potassium-sparing diuretic that blocks the epithelial sodium channel (ENaC) in the collecting duct.
• Mechanism in lithium-induced NDI:
- Lithium enters the collecting duct cells through ENaC.
- By blocking ENaC, amiloride reduces lithium uptake into the cells, thereby decreasing lithium-induced ADH resistance.
- Amiloride also has a mild diuretic effect (decreases urine output by reducing distal tubular sodium delivery).
• Dosing: 5-10 mg/day (adults).
• Combination: Often used with hydrochlorothiazide (HCTZ) for synergistic effect.
• Advantages: Prevents hypokalemia (unlike HCTZ alone).
• Limitations: May not fully reverse the polyuria; used as an adjunctive therapy.

✅ Model Answer:
• Genetic causes:
- X-linked (AVPR2 mutation) — most common congenital NDI.
- Autosomal recessive (AQP2 mutation).
- Autosomal dominant (AQP2 mutation, rare).
• Acquired causes:
- Drugs: Lithium (most common), amphotericin B, ifosfamide, foscarnet, demeclocycline, cisplatin.
- Metabolic: Hypercalcemia, hypokalemia.
- Renal: Sickle cell nephropathy, chronic kidney disease, obstructive uropathy, pyelonephritis.
- Other: Polycystic kidney disease, medullary sponge kidney, sarcoidosis, Sjögren syndrome.
- Post-obstructive: After relief of urinary obstruction.
• History: Lithium therapy is the most common drug-induced cause of NDI.

✅ Model Answer:
• Discontinue lithium: If possible, in consultation with psychiatry. Consider alternative mood stabilizers (valproate, lamotrigine, antipsychotics).
• If lithium must be continued:
- Minimize the dose to the lowest effective level.
- Monitor lithium levels closely.
• Pharmacological therapy:
- Amiloride: 5-10 mg/day (reduces lithium uptake into collecting duct cells).
- Hydrochlorothiazide (HCTZ): 25-50 mg/day (reduces urine volume by inducing mild hypovolemia).
- Combination: Amiloride + HCTZ is often used (prevents hypokalemia).
- Indomethacin: May be used (inhibits prostaglandins, enhances thiazide effect) — monitor renal function.
• Low-sodium diet: Reduces urine output.
• Avoid NSAIDs: Can worsen renal impairment.
• Hydration: Ensure free water access to prevent hypernatremia.

✅ Model Answer:
• Complications:
- Hypernatremia: Severe dehydration → confusion, seizures, coma, death.
- Chronic kidney disease: Long-term lithium use can cause tubulointerstitial nephritis and progressive CKD.
- Nephrogenic DI: May be irreversible if lithium is continued for a long time.
- Electrolyte imbalances: Hypokalemia (with thiazide use), hypercalcemia, hypomagnesemia.
- Lithium toxicity: Increased risk if dehydrated (reduced renal clearance).
- Nocturia: Sleep disturbance, fatigue.
- Psychosocial: Impact on school, social activities.
- Bipolar relapse: If lithium is discontinued without appropriate mood stabilization.

✅ Model Answer:
• Renal ultrasound: Important for evaluating structural damage in lithium-induced NDI.
• Findings:
- Nephrocalcinosis: Hyperechogenicity of the medullary pyramids (due to calcium deposition).
- Cysts: Microcysts or simple cysts (lithium can cause cyst formation).
- Small/echogenic kidneys: Chronic kidney disease (if long-term lithium use).
- Hydronephrosis: If obstructive uropathy is present.
• Indications:
- All patients with lithium-induced NDI should have a renal ultrasound to assess for nephrocalcinosis, cysts, and renal size.
- If nephrocalcinosis is found, check calcium, phosphate, PTH.
- If cysts are present, monitor renal function.
• Follow-up: Repeat ultrasound if renal function declines or if new symptoms develop.

✅ Model Answer:
• Prognosis:
- Reversible: If lithium is stopped early (<2 years of therapy), NDI may resolve.
- Irreversible: If lithium is continued for >10 years, NDI may become permanent (due to tubular atrophy and fibrosis).
- Renal function: May decline over time (lithium-induced interstitial nephritis).
- Bipolar disorder: Requires careful psychiatric management if lithium is stopped.
- Quality of life: Can be improved with amiloride and lifestyle modifications.
• Long-term follow-up:
- Monitor: Serum sodium, renal function, lithium levels.
- Nephrology: Regular follow-up for chronic kidney disease.
- Psychiatry: Manage bipolar disorder with alternative mood stabilizers.
- Education: Teach patient about the importance of hydration, signs of dehydration, and medication adherence.
- Lifestyle: Low-sodium diet, avoid dehydration, avoid NSAIDs.