Which of the following substances assists in the mechanical digestion of food in the stomach?

Gastric juice is essential for the mechanical digestion of food in the stomach. This complex fluid, which consists primarily of hydrochloric acid, pepsinogen, and intrinsic factor, plays several critical roles in digestion. The hydrochloric acid creates an acidic environment that helps break down food particles and activates the enzyme pepsin, which begins the digestion of proteins. Additionally, the churning action of the stomach, combined with the secretion of gastric juice, aids in mixing and breaking down food into a semi-liquid form called chyme. This process of mixing and grinding food not only provides a mechanical function but also prepares it for further digestion in the intestines.

Other substances mentioned do not primarily serve this mechanical function in the stomach. Saliva primarily aids in lubrication and carbohydrate digestion in the mouth. Bile is important for the digestion and absorption of fats, but it is released from the gallbladder into the small intestine, not the stomach. Salivary amylase, while a crucial enzyme for carbohydrate digestion, is active mainly in the mouth and does not contribute to mechanical processes in the stomach. Thus, gastric juice is specifically aligned with both mechanical and chemical digestion within that digestive organ.