Final answer:
The most advanced stage of iron deficiency anemia is marked by a hemoglobin level of 5 to 9 g/100 ml, indicating a serious depletion of iron and resulting in symptoms like fatigue and impaired development.
Step-by-step explanation:
The lab result that marks the most advanced stage of iron deficiency anemia is a significantly low hemoglobin level. In the context of iron deficiency anemia, this level is often reported to be between 5 to 9 g/100 ml of blood. This stage of anemia reflects a serious depletion of iron, affecting the ability of the blood to carry oxygen effectively. The symptoms associated with this advanced stage include extreme fatigue, pallor of the skin, poor appetite, and impaired growth and development, particularly noticeable in weaned infants and young children.