Which of the following buffer systems is not capable of adjusting the pH?

The correct answer is rooted in the understanding that the phrase "not capable of adjusting the pH" suggests a system that does not actively or effectively modulate acidity or alkalinity in response to changes in pH. Each of the listed buffer systems has a role in maintaining pH homeostasis within the body.

The phosphate buffer system operates primarily within the intracellular fluid and urine, where it can accept or donate protons to regulate pH. The respiratory compensation mechanism refers to the ability of the respiratory system to adjust levels of carbon dioxide, thus influencing blood pH. When carbon dioxide levels rise, the pH decreases, and vice versa; the lungs can increase ventilation to correct pH abnormalities. The hemoglobin buffer system functions in the blood, where hemoglobin can take up excess hydrogen ions (decreasing acidity) or release them (increasing acidity) depending on the surrounding pH level.

Since all these systems have mechanisms to respond to changes in pH, it is justifiable to say that they are capable of increasing or lowering pH, making the choice that implies they do not have that capacity incorrect. In essence, the phrase in the question reflects a misunderstanding of the roles these buffer systems play in maintaining acid-base balance in the body.