Which of the following allows the passage of small amounts of urea into medullary interstitium?

The segment of nephron that allows the passage of small amounts of urea into medullary interstitium is collecting duct.

Urea diffuses into the tubular fluid in thin ascending limbs of the long nephron loops when its concentration rises in the medullary interstitium. The cells of thick ascending limb, distal convoluted tubule, and cortical portion of the collecting duct are impermeable to urea and so it remains in the filtrate. The water reabsorption continues throughout these regions of nephron (especially due to ADH). This water reabsorption further increases the concentration of urea in the tubular fluid. In the medullary region the urea diffuses into the interstitial fluid and the cycle repeats. This phenomenon is urea recycling.