Accumulation of which of the following molecules in the muscles occurs as a result of vigorous exercise?

During vigorous exercise, the muscles require a lot of energy quickly. This energy comes from the breakdown of glucose through a process called glycolysis. In glycolysis, glucose is converted to pyruvic acid (or pyruvate), producing some ATP (energy). Normally, when oxygen is available (aerobic conditions), pyruvic acid enters the mitochondria and is further broken down to produce more ATP.

However, during intense exercise, the oxygen supply to the muscles becomes insufficient (anaerobic conditions). In this situation, pyruvic acid cannot be processed further in the mitochondria. Instead, it is converted to L-lactic acid (also called lactate) right in the muscle cells. This conversion happens through a reaction catalyzed by the enzyme lactate dehydrogenase. The reaction is:

$$ \text{Pyruvic acid} + \text{NADH} \rightarrow \text{L-lactic acid} + \text{NAD}^+ $$

This step regenerates NAD⁺, which is necessary for glycolysis to continue producing ATP without oxygen. As a result, L-lactic acid builds up in the muscles because it is produced faster than it can be removed or transported away.

Now, let's examine the other options:

Option A: Glycogen - Glycogen is the stored form of glucose in muscles and liver. During exercise, glycogen is broken down to provide glucose for energy production. So, glycogen levels decrease, not accumulate.

Option B: Pyruvic acid - Pyruvic acid is an intermediate in glycolysis. Under anaerobic conditions, it is rapidly converted to L-lactic acid. Therefore, it does not accumulate; instead, it is consumed.

Option C: Glucose - Glucose is the starting fuel for glycolysis. It is used up during exercise to produce energy, so its levels decrease in the muscles.

Option D: L-lactic acid - As explained, under anaerobic conditions during vigorous exercise, pyruvic acid is converted to L-lactic acid, leading to its accumulation. This buildup causes muscle fatigue and the burning sensation felt after intense exercise.

Hence, the correct answer is Option D.