What condition occurs when there is not enough iron to produce hemoglobin?

Iron deficiency anaemia occurs when there is an insufficient amount of iron in the body to produce hemoglobin, which is the protein in red blood cells responsible for transporting oxygen. When the body lacks adequate iron, it cannot generate enough hemoglobin, leading to a decreased number of red blood cells and resulting in symptoms such as fatigue, weakness, and pallor. This condition is particularly common and can arise from various factors such as poor dietary intake, loss of blood, or increased demand during periods of growth or pregnancy.

In contrast, thalassemia is a genetic blood disorder characterized by the production of abnormal hemoglobin, which is unrelated to iron levels. Vitamin B12 deficiency affects red blood cell formation, but it does so through the lack of vitamin B12 rather than iron. Polycythemia involves the overproduction of red blood cells and is not connected to iron deficiency or hemoglobin production directly. This highlights the distinct nature of iron deficiency anaemia, making it the correct answer for the given question.