**Question:** 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 liters. Investigation showed Na 130 mEq/L, K - 5 mEq/L, urea 15mg/dL, sugar - 65 mg/dL. The plasma osmolality is 268 mosmol/L and urine osmolality is 45 mosmol/L. The most likely diagnosis is-

A. Diabetes insipidus
B. Diabetes mellitus
C. Primary aldosteronism
D. Hypertension

**Correct Answer:** A. Diabetes insipidus

**Core Concept:**

Diabetes insipidus and diabetes mellitus are both disorders characterized by excessive urine production. However, they differ in their etiology, clinical presentation, and laboratory findings. Diabetes insipidus results from impaired secretion or action of antidiuretic hormone (ADH) or vasopressin, while diabetes mellitus is due to impaired glucose tolerance.

**Why the Correct Answer is Right:**

In this case, the patient presents with polyuria (excessive urine production) and polydipsia (excessive thirst). The plasma osmolality is 268 mosmol/L, which is significantly elevated. Urine osmolality is also elevated at 45 mosmol/L. These values indicate hypernatremia and hypo-osmolality.

**Why Other Options are Incorrect:**

B. Diabetes mellitus:
1. In diabetes mellitus, glucose levels in blood and urine are elevated due to impaired glucose tolerance. The patient would have hyperglycemia (high blood glucose levels) and glucosuria (glucose in the urine). The plasma osmolality would be normal or slightly elevated, and urine osmolality would be elevated due to increased glucose concentration in urine.

C. Primary aldosteronism:
1. Aldosterone is a hormone secreted by the adrenal cortex that promotes sodium reabsorption in the distal nephron and potassium secretion. Patients with primary aldosteronism present with hypokalemia (low potassium levels), hyperkalemic acidosis, and hyperkalemic alkalosis. The plasma osmolality would be normal or slightly elevated, and urine osmolality would be elevated due to increased sodium concentration in urine.

D. Hypertension:
1. Hypertension is defined as elevated blood pressure. The patient would not present with polyuria and polydipsia unless secondary hypertension is causing inappropriate antidiuretic hormone (ADH) secretion. In this case, the patient would have normal blood pressure and normal urine osmolality.

In summary, the correct diagnosis is diabetes insipidus, which is a type of diabetes mellitus characterized by excessive urine production secondary to impaired secretion or action of ADH (antidiuretic hormone). The elevated plasma osmolality and normal urine o