Carcinogen is defined as

• A. A toxin that damages the liver
• B. A substance that prevents cancer
• C. A virus that causes infection
• D. A cancer-causing substance

Answer: (D)

A carcinogen is any substance that has the potential to cause cancer by promoting malignant changes in cells. It can be a chemical, a form of radiation, or even certain viruses that contribute to cancer development. The statement describing a cancer-causing substance fits this definition precisely. The other ideas don’t apply: something that damages the liver is a hepatotoxin, not necessarily a cancer-causing agent; something that prevents cancer is anti-carcinogenic; and a virus that causes infection is a pathogen, which may or may not be carcinogenic depending on the context.