Final answer:
A high-protein diet is most likely to cause a prerenal plasma urea increase, which occurs before the blood reaches the kidneys due to the increased production of urea associated with higher protein metabolism.
Step-by-step explanation:
A high-protein diet may lead to an increase in plasma urea levels. When considering the causes of variation in blood urea level, we typically categorize increased urea levels into pre-renal, renal, and post-renal categories. A high intake of protein could potentially result in an increased metabolic workload for the kidneys to excrete nitrogenous waste, such as urea, leading to a prerenal plasma urea increase.
Pre-renal uremia can occur when there is increased production of urea in the body due to factors such as high protein intake or decreased excretion due to conditions like dehydration. This type of uremia occurs before the blood reaches the kidneys. Therefore, an excess of protein in the diet would not be expected to lead to a decrease in plasma urea but rather an increase, specifically a pre-renal increase, due to the increased load of urea that the kidneys need to filter out of the blood.
In conclusion, the correct answer to the question is a. prerenal plasma urea increase.