Question

serum is essentially identical to plasma except for the absence of _______.

Explanation:

In the field of Biology (a subfield of Natural Science), serum and plasma are components of blood. Plasma contains fibrinogen (and other clotting factors), while serum is plasma from which fibrinogen (and clotting factors) have been removed, typically because the blood has clotted. So the key difference is the absence of fibrinogen (or clotting factors, with fibrinogen being a major one) in serum compared to plasma.

Answer:

fibrinogen (or clotting factors)