**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. Hyponatremia
B. Hypernatremia
C. Hyperkalemia
D. Hypokalemia

**Answer Explanation:**

**Core Concept:**

In this case, we are dealing with a patient presenting with symptoms of excessive fluid loss (polyuria) and thirst (polydipsia), which are clinical manifestations of a disturbance in electrolyte balance. The term for this condition is "hypertonic dehydration" or "hyponatremic hypoosmolar non-anhidrotic hyponatremia." This refers to a situation where the plasma osmolality is high (268 mosmol/L), while the urine osmolality is low (45 mosmol/L). In such cases, water is being lost from the body due to high plasma osmolality, leading to the above symptoms.

**Why the Correct Answer is Hyponatremia (Option A):**

The correct answer is Hyponatremia because it is the primary disturbance in this case. Hyponatremia occurs when the serum sodium concentration is low. In the provided case, the sodium level is 130 mEq/L, which is below the normal range (135-145 mEq/L). Additionally, the plasma osmolality is high (268 mosmol/L), indicating the loss of water in the body due to high plasma osmolality.

**Why Other Options are Incorrect:**

**Option B (Hypernatremia):**

The patient is not suffering from hypernatremia, which is characterized by high serum sodium levels (>145 mEq/L). The patient has a sodium level of 130 mEq/L, which is below the normal range, indicating hyponatremia.

**Option C (Hyperkalemia):**

The patient does not have hyperkalemia, which is characterized by high serum potassium levels (>5.0 mEq/L). The potassium level in this patient is 4.5 mEq/L, which is within the normal range (3.5-5.0 mEq/L).

**Option D (Hypokalemia):**

The patient is not presenting with hypokalemia, which is characterized by low serum potassium levels (<3.5 mEq/L). The potassium level in this patient is 4.5 mEq/L, which is