Which of the following is primarily collected for biochemical analysis?

The correct answer is serum, which is primarily collected for biochemical analysis because it contains a wide range of substances that can provide valuable information about the body's metabolic state and organ function. Serum is obtained after blood has clotted and has been centrifuged to separate the liquid component from the cellular components, which means it contains various proteins, enzymes, hormones, and electrolytes that are crucial for many biochemical tests.

Biochemical analyses often measure levels of substances like glucose, electrolytes, and hormones, and these tests require the absence of clotting factors, which is why serum is preferred. It provides a clearer picture of the biochemical composition of the blood without the interference of clotting mechanisms.

While whole blood, capillary blood, and plasma can be used for certain tests, they are generally not as suitable for comprehensive biochemical analysis as serum. Whole blood retains the cells and clotting factors that may interfere with some biochemical tests, capillary blood is a smaller sample often used in rapid tests or monitoring scenarios, and plasma contains clotting factors since it is derived from whole blood that has been treated to prevent clotting. Thus, serum is distinctly favored when the goal is to analyze biochemical markers.