**Core Concept**
Secondary bile acids are produced by the action of intestinal bacteria on primary bile acids, such as cholic acid and chenodeoxycholic acid. This process involves the dehydroxylation and epimerization of primary bile acids, resulting in the formation of new bile acids with different properties.

**Why the Correct Answer is Right**
The correct answer is deoxycholic acid, which is formed by the action of intestinal bacteria on cholic acid. This process involves the removal of a hydroxyl group from cholic acid, resulting in the formation of deoxycholic acid. Deoxycholic acid is a secondary bile acid that plays a crucial role in the enterohepatic circulation of bile acids and has been implicated in the pathogenesis of certain gastrointestinal disorders.

**Why Each Wrong Option is Incorrect**
**Option A:** Cholic acid is a primary bile acid, not a secondary bile acid. It is synthesized in the liver and secreted into the bile, where it is then modified by intestinal bacteria to form secondary bile acids like deoxycholic acid.
**Option B:** Chenodeoxycholic acid is also a primary bile acid, which is synthesized in the liver and secreted into the bile. While it can be converted to ursodeoxycholic acid by intestinal bacteria, it is not a secondary bile acid itself.
**Option C:** Ursodeoxycholic acid is a secondary bile acid, but it is formed by the action of intestinal bacteria on chenodeoxycholic acid, not cholic acid. It is used as a medication to dissolve cholesterol gallstones and improve liver function in certain conditions.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that secondary bile acids, like deoxycholic acid, can be toxic to the colon and have been implicated in the pathogenesis of certain gastrointestinal disorders, such as colon cancer and inflammatory bowel disease.

**Correct Answer:** C. Deoxycholic acid