What condition is characterized by a lower than normal red blood cell count?

Anemia is characterized by a lower than normal red blood cell count, which leads to reduced oxygen delivery to tissues throughout the body. This condition can arise from several causes, including nutritional deficiencies (such as iron, vitamin B12, or folate), bone marrow disorders, chronic diseases, or blood loss. The reduced number of red blood cells can result in symptoms such as fatigue, weakness, pallor, and shortness of breath due to the body's inability to transport adequate oxygen.

In contrast, leukopenia refers to a decreased white blood cell count, which affects the body's ability to fight infections, while leukocytosis involves an increased white blood cell count, often indicating an infection or other medical condition. Sickle cell disease is a genetic disorder that affects the shape of red blood cells, leading to various complications, but it does not solely define the condition of having a lower red blood cell count independent of the disease's complexities. Hence, anemia directly relates to and is characterized by the low red blood cell count, making it the correct choice.