Which pregnancy category has clear evidence of causing abnormalities in the fetus?

The category that indicates clear evidence of causing abnormalities in the fetus is indeed X. This classification signifies that studies in animals or humans have demonstrated fetal abnormalities, and the risks involved in using these medications far outweigh any potential benefits during pregnancy. Medications categorized as X are contraindicated in pregnant women due to the significant potential for harm to the fetus.

In contrast, the other categories (A, B, C) do not represent the same level of risk. Category A indicates that there is no risk to the fetus based on controlled studies, Category B suggests that animal studies have shown no risk but lack adequate human studies, and Category C signifies that risks cannot be ruled out; however, the benefits may still be acceptable in certain situations despite the potential risks. Understanding these categories helps healthcare providers make informed decisions when prescribing medications to pregnant patients, ensuring the safety of both the mother and the developing fetus.