Identify a common side effect of chemotherapy in small animals.

Chemotherapy in small animals often targets rapidly dividing cells, which include not only cancer cells but also healthy cells such as those in the bone marrow. Bone marrow suppression is a common side effect of chemotherapy, leading to a decreased production of blood cells. This can result in various complications, including anemia, increased susceptibility to infections due to lower white blood cell counts, and difficulty in blood clotting resulting from reduced platelets.

Understanding this side effect is crucial for managing patients undergoing chemotherapy, as veterinarians need to monitor blood cell levels and may need to provide supportive care if significant suppression occurs. In contrast, the other options do not typically align with the known effects of chemotherapy, where increased appetite and weight gain are not commonly observed, and improvements in energy levels are unlikely due to the general malaise that can accompany treatment.