What condition is characterized by a reduced ability of red blood cells to deliver oxygen to tissues?

Anemia is characterized by a reduced ability of red blood cells to deliver oxygen to tissues, which is primarily due to a decrease in the number of red blood cells or the amount of hemoglobin they contain. Hemoglobin is the protein in red blood cells responsible for binding oxygen and transporting it throughout the body. When anemia occurs, this transport function becomes compromised, leading to symptoms such as fatigue, weakness, and pallor due to insufficient oxygen supply to the tissues.

In contrast, leukemia is a type of cancer that affects blood cells, particularly white blood cells, and is not primarily related to oxygen transport. Hemophilia is a bleeding disorder that affects the blood's ability to clot, rather than the delivery of oxygen. Polycythemia involves an increase in red blood cell mass, which can lead to thickened blood but does not reflect the condition described in the question. Thus, the characteristics of anemia align directly with the definition provided in the question.