This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Respiratory system, Subject name: Physiology
In which of the following disease normal structure of hemoglobin but their blood volume reduced in? 
Select and explain. 
A. Hemorrhage, B. Sickle - cell anemia, C. DIC, D. Thalassemia
D: Thalassemia
Answer: D: Thalassemia 
Explanation: Thalassemia describes a group of inherited disorders characterized by reduced or absent amounts of hemoglobin, the oxygen-carrying protein inside the red blood cells. There are two basic groups of thalassemia disorders: alpha thalassemia and beta thalassemia. These conditions cause varying degrees of anemia, which can range from insignificant to life-threatening.Ref: Ganong&;s Review of Medical Physiology; 24th edition