A 64-year-old man has severe polyuria and polydipsia, drinking 3 to 4 glasses of water and producing over 0.5 liters of urine each hour. Rajesh a new medicine resident places the patient on overnight water restriction for fuher analysis. The test results shown below were obtained the following morning. Plasma sodium concentration: 155 mEq/L Urine osmolarity: 90 mOsmol/L Urine glucose concentration: 0 mg/dL Which of the following is the most likely diagnosis?

**Core Concept**
The patient's symptoms of polyuria and polydipsia, along with the laboratory results, suggest a disorder of water and electrolyte balance. The plasma sodium concentration is elevated, indicating hypernatremia, while the urine osmolarity is low, suggesting an inability to concentrate urine. This combination of findings points to a problem with the regulation of antidiuretic hormone (ADH) or the kidneys' response to ADH.

**Why the Correct Answer is Right**
The patient's symptoms and laboratory results are consistent with the diagnosis of central diabetes insipidus (CDI). CDI is caused by a deficiency of ADH, which is produced by the hypothalamus and released by the posterior pituitary gland. ADH plays a crucial role in regulating water reabsorption in the kidneys, and a deficiency leads to an inability to concentrate urine. The patient's low urine osmolarity and hypernatremia support this diagnosis. Additionally, the absence of glucose in the urine suggests that the patient's kidneys are not responding to ADH, as glucose would be present in the urine if the kidneys were not responding to ADH.

**Why Each Wrong Option is Incorrect**
**Option A:** Nephrogenic diabetes insipidus is a possible diagnosis, but the patient's symptoms and laboratory results are more consistent with CDI. In nephrogenic diabetes insipidus, the kidneys are unable to respond to ADH, but the plasma sodium concentration is typically not as elevated as in CDI.

**Option B:** Psychogenic polydipsia is a possible diagnosis, but the patient's symptoms and laboratory results are more consistent with CDI. In psychogenic polydipsia, the patient drinks excessive amounts of water due to a psychiatric disorder, but the plasma sodium concentration would not be as elevated as in CDI.

**Option C:** Primary polydipsia is a possible diagnosis, but the patient's symptoms and laboratory results are more consistent with CDI. In primary polydipsia, the patient drinks excessive amounts of water due to a psychiatric disorder, but the plasma sodium concentration would not be as elevated as in CDI.

**Option D:** Is a possible choice but the provided information does not support it.

**Clinical Pearl / High-Yield Fact**
In patients with CDI, the plasma sodium concentration can become severely elevated, leading to complications such as seizures and coma. It is essential to diagnose and treat CDI promptly to prevent these complications.

**Correct Answer:** C. Central diabetes insipidus.

✓ Correct Answer: C. Diabetes insipidus