Which of the following is considered a normal form of hemoglobin?

Deoxyhemoglobin is considered a normal form of hemoglobin because it represents the state of hemoglobin when it has released oxygen to the tissues. Hemoglobin exists in different forms depending on its oxygenation status, and deoxyhemoglobin specifically refers to hemoglobin that is not bound to oxygen. This form plays a crucial role in the transport of carbon dioxide from tissues back to the lungs, demonstrating the dynamic nature of hemoglobin in oxygen delivery and carbon dioxide removal.

In contrast, carboxyhemoglobin occurs when hemoglobin binds to carbon monoxide instead of oxygen, which can be harmful and is not a normal physiological state. Methemoglobin is formed when the iron in hemoglobin is oxidized to the ferric state, preventing it from carrying oxygen effectively, and sulfhemoglobin is produced when sulfur binds to hemoglobin, interfering with its ability to transport oxygen. These latter forms indicate some form of dysfunction or abnormal binding that is not desired in healthy biological systems, highlighting why deoxyhemoglobin is the only one among the options that represents a normal, functional aspect of hemoglobin.