In a patient with severe central diabetes insipidus caused by a lack of antidiuretic hormone secretion, which part of the tubule would have the lowest tubular fluid osmolarity?
**Core Concept**
The question is testing the understanding of the role of antidiuretic hormone (ADH), also known as vasopressin, in regulating water reabsorption in the kidneys. ADH stimulates the insertion of aquaporin-2 water channels in the collecting ducts, which increases water reabsorption and concentrates the urine. In central diabetes insipidus, the lack of ADH secretion impairs this process, leading to an inability to concentrate urine.
**Why the Correct Answer is Right**
In the collecting ducts, ADH stimulates the insertion of aquaporin-2 water channels, which increases water reabsorption and concentrates the urine. Without ADH, the collecting ducts would have a low reabsorption of water, resulting in a lower tubular fluid osmolarity compared to other parts of the nephron. The collecting ducts are responsible for the final concentration of the urine, and a lack of ADH would impair this process, leading to a lower osmolarity in this region.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option might be tempting, but the loop of Henle is not the primary site affected by ADH. The loop of Henle is responsible for the reabsorption of ions and water, but ADH's primary effect is on the collecting ducts.
* **Option B:** The proximal convoluted tubule is responsible for the bulk reabsorption of glucose, amino acids, and other substances, but it is not directly affected by ADH. The lack of ADH would not primarily affect the osmolarity in this region.
* **Option C:** The distal convoluted tubule is involved in the fine-tuning of ion and water reabsorption, but it is not the primary site affected by ADH. The collecting ducts are more directly involved in the process.
**Clinical Pearl / High-Yield Fact**
In central diabetes insipidus, the administration of desmopressin (a synthetic analogue of ADH) can help to concentrate the urine and alleviate symptoms. This is a key treatment strategy for this condition.
**Correct Answer: C.** Collecting ducts.