After several miles of running and sprinting in intervals, the runner’s muscles ache and feel fatigued. Which is the best explanation for what has occurred?
The runner's muscles have accumulated lactic acid.
The runner’s body has failed to perform cellular respiration.
The runner’s cells are relying completely on aerobic respiration.
The runner’s muscles have accumulated carbon dioxide.

When the body is subjected to vigorous exercise, lactic acid tends to build up in the muscles, hence, the most probable explanation for the fatigue felt by the runner is the accumulation of lactic acid in the runner's muscle

When the body is subjected to vigorous exercise such as running several miles as performed by the subject in the question, lactic acid accumulates temporarily in the muscles.

This temporary build up of lactic acid in the muscles causes feeling of fatigue or weakness as the body tends to become short of oxygen required to breakdown the glucose in the blood.

Therefore, the runner most probably has accumulated lactic acid in his muscles.

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