What condition is characterized by a decrease in certain elements of the blood, especially red cells and hemoglobin?

The condition characterized by a decrease in certain elements of the blood, particularly red blood cells and hemoglobin, is anemia. Anemia is defined as a deficiency in the number of red blood cells or the amount of hemoglobin they contain, which leads to reduced oxygen-carrying capacity of the blood. This can result from various causes, such as iron deficiency, vitamin B12 deficiency, or chronic diseases.

In contrast, dehydration primarily affects the fluid balance in the body rather than the actual number of blood cells. Leukemia involves the overproduction of abnormal white blood cells, not a reduction in red blood cells or hemoglobin. Thrombocytopenia is specifically a condition that results in low platelet counts and does not address red blood cells or hemoglobin levels. Therefore, anemia is uniquely defined by a decrease in red blood cells and hemoglobin, making this the correct answer.