Antimetabolites like methotrexate mainly cause which of the following side effects?

Antimetabolites, such as methotrexate, primarily work by interfering with the synthesis of DNA and RNA, which is essential for cell division. This mechanism is particularly impactful on rapidly dividing cells, such as those found in the bone marrow, where blood cell production occurs. As a result, one of the significant side effects of methotrexate is myelosuppression, characterized by a decrease in the production of blood cells. This leads to conditions such as anemia, increased risk of infection, and bleeding tendencies due to low levels of red blood cells, white blood cells, and platelets.

In contrast, the other options do not align with the expected effects of antimetabolites. Enhanced energy levels, weight gain, and improved organ function are not typical outcomes of treatment with methotrexate. Instead, patients may experience fatigue due to anemia and other complications associated with myelosuppression, which underscores the importance of monitoring blood counts during therapy.