A 45-year old woman is admitted to the hospital following a head injury. She has severe polyuria (producing 1 L of urine every 2 hours) and polydipsia (drinking 3 to 4 glasses of water every hour).During a 24-hour period in the hospital, the woman produces 10 L of urine, containing no glucose. She is placed on overnight water restriction for fuher evaluation. The following morning, she is weak and confused. Her serum osmolarity is 330mOsm/L, her serum is 164 mEg/L, and her urine osmolarity is 70 mOsm/L. She is treated with dDAVP by nasal spray. Within 24 hours of initiating the treatment, her serum osmolarity is 295mOsm/L and her urine osmolarity is 620mOsm/L.

**Core Concept**
The patient's symptoms of polyuria and polydipsia, along with the laboratory findings of low serum osmolality and high urine osmolality, suggest a disorder of water reabsorption in the kidneys. This condition is characterized by an inability to concentrate urine, leading to excessive water loss.

**Why the Correct Answer is Right**
The patient's condition is consistent with central diabetes insipidus (CDI), a rare disorder caused by the deficiency of antidiuretic hormone (ADH), also known as vasopressin. ADH plays a crucial role in regulating water reabsorption in the collecting ducts of the kidneys. Without sufficient ADH, the kidneys are unable to concentrate urine, resulting in the excessive water loss and symptoms exhibited by the patient. The administration of desmopressin (dDAVP) by nasal spray increases the levels of ADH in the body, allowing the kidneys to reabsorb water and concentrate urine.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not address the underlying cause of the patient's symptoms. While hypernatremia is a complication of CDI, it is not the primary diagnosis.
* **Option B:** This option is incorrect because it is a treatment for diabetes mellitus, not CDI. The patient's urine contains no glucose, ruling out diabetes mellitus as a diagnosis.
* **Option C:** This option is incorrect because it is a treatment for SIADH (Syndrome of Inappropriate Antidiuretic Hormone Secretion), a condition characterized by excessive ADH production. In contrast, the patient's symptoms suggest a deficiency of ADH.

**Clinical Pearl / High-Yield Fact**
In patients with CDI, the administration of dDAVP can lead to a rapid improvement in symptoms, but it is essential to monitor serum osmolality and urine output to avoid water intoxication. This is a classic exam trap, as patients with CDI may initially present with hypernatremia due to excessive water loss, but treatment with dDAVP can lead to hyponatremia if not carefully managed.

**Correct Answer: D. Central Diabetes Insipidus**

✓ Correct Answer: C. Central Diabetes Insipidus