After exercising, theo is aware that anaerobic respiration occurred in his body because his muscles ache. which substance caused his muscles to feel sore?
lactic acid
glucose
carbon dioxide
glycogen

The correct answer is lactic acid.

During vigorous exercise such as weight lifting, swimming etc the level of oxygen decreases in the muscle cells due to which complete oxidation of glucose could not take place.

Instead, the pyruvate is converted into lactate in presence of lactate dehydrogenase. It produces NAD⁺ being lost during the process of glycolysis.

Lactic acid can cause muscle discomfort and muscle soreness. However, it is not the only culprit responsible for delayed onset muscle soreness (DOMS).

Other factors may include swelling which is caused by the influx of prostaglandins, WBCs, and nutrients in muscle cells in order to repair the damage caused by exercise.