**Central Diabetes Insipidus (CDI)**

**Core Concept**
Central Diabetes Insipidus (CDI) is a rare disorder characterized by the inability to regulate body fluids due to a deficiency in antidiuretic hormone (ADH), also known as vasopressin. ADH is produced in the hypothalamus and released by the posterior pituitary gland, playing a crucial role in water reabsorption in the kidneys.

**Why the Correct Answer is Right**
The patient's symptoms of polydipsia and polyuria, along with a low plasma osmolality and a low urine osmolality, are indicative of CDI. The absence of postural drop in blood pressure suggests that the patient is not experiencing volume depletion, which is often seen in primary polydipsia. The low plasma osmolality and low urine osmolality are consistent with a defect in ADH production or release. The normal blood pressure and lack of postural drop also make primary polydipsia less likely.

**Why Each Wrong Option is Incorrect**
**Option A:** Primary Polydipsia - This condition is characterized by excessive fluid intake leading to polyuria, but it is usually associated with a high plasma osmolality and a low urine osmolality. This patient's low plasma osmolality and low urine osmolality make primary polydipsia less likely.
**Option B:** Nephrogenic Diabetes Insipidus (NDI) - This condition is characterized by a failure of the kidneys to respond to ADH, leading to polyuria and polydipsia. However, NDI is usually associated with a normal or elevated plasma osmolality and a low urine osmolality. This patient's low plasma osmolality makes NDI less likely.
**Option C:** Diabetes Mellitus - This condition can cause polyuria and polydipsia, but it is usually associated with hyperglycemia and a high plasma osmolality. This patient's normal blood sugar levels and low plasma osmolality make diabetes mellitus less likely.

**Clinical Pearl / High-Yield Fact**
CDI can be caused by head trauma, which may damage the hypothalamus or posterior pituitary gland, leading to ADH deficiency. It is essential to consider CDI in patients with a history of head trauma or other central nervous system disorders.

**Correct Answer: C. Diabetes Mellitus**