USMLE (United States Medical Licensing Examination) Step 1 Practice Exam

What is a key characteristic of the inner medullary collecting tubules in response to ADH?

1. Increased sodium retention

2. Increased urea permeability

3. Decreased urine volume

4. Decreased interstitium tonicity

The correct answer is: Increased urea permeability

The inner medullary collecting tubules are influenced by antidiuretic hormone (ADH), which plays a significant role in regulating water balance in the body. When ADH is present, one of its key actions is to increase the permeability of the collecting ducts to urea. This is accomplished through the upregulation of urea transporters in the inner medullary collecting duct, allowing for enhanced reabsorption of urea from the tubular fluid back into the medullary interstitium. This increase in urea permeability not only helps to conserve water but also contributes to the creation of a hyperosmotic interstitium necessary for water reabsorption. This process is important for concentrating urine and maintaining the body's fluid balance. While options involving sodium retention and decreased urine volume are relevant to the overall effects of ADH, they do not specifically address the key characteristic of urea permeability within the inner medullary collecting tubules. Therefore, recognizing that ADH's impact on urea permeability is central to its function in the kidneys supports the correctness of the answer provided.