What is the only fuel source that can be used during anaerobic metabolism?

In the context of anaerobic metabolism, the primary fuel source utilized is sugar, specifically in the form of glucose. Anaerobic metabolism occurs when the body requires energy production without the presence of oxygen, typically during high-intensity activities where oxygen demand exceeds supply.

During this process, glucose is broken down through anaerobic glycolysis, which generates ATP (adenosine triphosphate) for energy. The end product of this metabolic pathway is lactic acid, which can accumulate in the muscles during intense exercise. Other fuel sources, such as fats and proteins, require a presence of oxygen and undergo aerobic metabolism, making them unsuitable for quick energy production in the absence of oxygen.

Therefore, the correct answer identifies sugar as the only fuel source that can actively participate in anaerobic metabolism, highlighting its essential role in providing immediate energy during high-intensity activities.