Final answer:
The measurement of calcium is affected by a patient's position due to its relationship with plasma proteins that can be influenced by changes in positioning. Therefore, the correct answer is B. Calcium.
Step-by-step explanation:
The test that is affected by a patient's position is the measurement of calcium. Specifically, total calcium levels can be influenced by a patient's position due to the relationship between calcium and plasma proteins. As half of the blood calcium (non-diffusible) is bound to plasma proteins, a change in positioning can alter the blood flow and concentration of proteins, thus affecting the calcium measurement. Albumin, although it is the most abundant plasma protein and influences the movement of water in and out of blood vessels, is not affected in its measurement by patient position. Enzymes, such as those measured in liver function tests or pancreatic enzymes like amylase and lipase, are also not typically affected by the patient's position during blood sample collection.