Number of aquaporins cloned is:

**Core Concept**
Aquaporins are a family of proteins that form water channels, allowing for the selective transport of water across cell membranes. They play a crucial role in maintaining proper water balance and osmotic pressure in various tissues and organs.

**Why the Correct Answer is Right**
Aquaporins have been extensively studied, and numerous isoforms have been cloned, characterized, and expressed in various organisms. The human genome encodes 13 aquaporin genes, which are divided into two subfamilies: the aquaporin (AQP) family and the aquaglyceroporin family. Each aquaporin isoform has a distinct tissue distribution, facilitating the transport of water, glycerol, or other small solutes across cell membranes.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not accurately reflect the number of aquaporin isoforms cloned and characterized.
**Option B:** This option is incorrect as the actual number of aquaporin genes in the human genome is higher than the number specified.
**Option C:** This option is incorrect as it is lower than the actual number of aquaporin genes in the human genome.

**Clinical Pearl / High-Yield Fact**
Aquaporins play a crucial role in various physiological processes, including water reabsorption in the kidneys, water transport across the blood-brain barrier, and glycerol transport in adipose tissue. Dysfunction of aquaporins has been implicated in various diseases, including nephrogenic diabetes insipidus and edema.

**Correct Answer: D. 13**