An acid-base disturbance occurs when buffering capacity is exceeded.

• A. When the pH remains exactly 7.0.
• B. When buffering capacity is exceeded.
• C. Only due to vomiting.
• D. Only due to kidney failure.

Answer: (B)

The key idea is that buffers act as the first line of defense by neutralizing excess hydrogen ions or hydroxide ions to keep pH stable. But buffers have a finite capacity. When the amount of acid or base entering the system overwhelms these buffering systems, they can no longer maintain pH, and the pH then moves away from normal, producing an acid-base disturbance. That’s why the statement about buffering capacity being exceeded correctly identifies the trigger for a disturbance: once buffering is overwhelmed, pH shifts toward acidosis or alkalosis depending on the direction of the load. The other ideas are not sufficient reasons on their own—pH being exactly 7.0 isn’t required for a disturbance (normal is roughly 7.35–7.45), and disturbances can arise from many causes beyond vomiting or kidney failure.