Which substance is NOT found in blood serum?

Blood serum is the clear, yellowish fluid that remains after blood has clotted and the cells and clotting factors have been removed. It contains a variety of substances, including glucose, electrolytes, hormones, antibodies, and waste products.

Coagulation proteins, such as fibrinogen, are essential for the clotting process; however, they are not present in serum. When blood coagulates, these proteins are consumed in the formation of the clot, which is why they are absent in serum. Therefore, the correct answer identifies a substance that is specifically not found in blood serum due to its role in clot formation.

Glucose and electrolytes are typically present in serum and are commonly analyzed in laboratory tests to assess metabolic and electrolyte balance. Blood gas values, on the other hand, are usually measured in whole blood that has not been allowed to clot, as they provide information about respiratory and metabolic function directly from the blood without the interference of the clotting process.