Which condition is characterized by low levels of hemoglobin in the blood?

The condition characterized by low levels of hemoglobin in the blood is anemia. Hemoglobin is the protein in red blood cells responsible for transporting oxygen throughout the body. When hemoglobin levels are low, the body can struggle to deliver sufficient oxygen to tissues, leading to various symptoms such as fatigue, weakness, and pallor. Anemia can be caused by a range of factors including nutritional deficiencies (like iron or vitamin B12), chronic diseases, or bone marrow problems.

In contrast, thrombocytopenia refers to a low platelet count, which affects blood clotting rather than oxygen transport. Polycythemia is characterized by an excess of red blood cells, leading to higher than normal levels of hemoglobin. Leukemia involves the uncontrolled production of abnormal white blood cells, impacting the body's ability to fight infections and potentially affecting red blood cell production, but it is not defined by low hemoglobin levels specifically. Understanding these distinctions clarifies why anemia is the correct choice for this question.