A 33-year-old lady presents with polydipsia and polyuria. Her symptoms started soon after a road traffic accident 6 months ago. The blood pressure is 120/80 mm Hg with no postural drop. The daily urinary output is 6-8 litres. Investigation showed, Na 130 mEq/L, K.3.5 mEq/L, urea 15mgAIL, sugar-65 mg/dL. The plasma osmolality is 268 mosmoUL and urine osmolality 45 mosmol/L. The most likely diagnosis is -
First lets go through the normal values of the indices required to solve this question​

Serum Sodium (Na) → 136-145 meq/L
Serum Potassium → 3.5-5.0 meq/L 
Serum Osmolality → 285-295 mmol/kg 
Serum Urea → 10-20 mg/dL 350
Urine Osmolality → 350 -1000 mosmol/kg.

Let's see the values in the patient

- Serum sodium        →     decreased
- Serum Osmolality   →   decreased
- Urine Osmolality     →  decreased
Central diabetes insipidus
Nephrogenic diabetes insipidus
Resolving acute tubular necrosis
Psychogenic polydipsia