Which condition is characterized by a decreased number of red blood cells?

Anemia is characterized by a decreased number of red blood cells in the body. This condition can result from various factors including nutritional deficiencies (such as iron, folate, or vitamin B12), chronic diseases, or genetic disorders. Red blood cells are crucial for transporting oxygen from the lungs to the rest of the body and returning carbon dioxide from the body back to the lungs. When their numbers decrease, it leads to a reduced capacity to carry oxygen, resulting in symptoms like fatigue, weakness, and paleness.

Hypertension refers to high blood pressure and does not directly relate to the number of red blood cells. Leukemia is a type of cancer affecting the blood and bone marrow that usually leads to an increase in abnormal white blood cells rather than a decrease in red blood cells. Thrombocytopenia is characterized by a low platelet count, which affects clotting, but does not specifically denote changes in the red blood cell count. Therefore, anemia is the clear choice that directly correlates with a decrease in red blood cells.