Which of the following organs is not directly involved in the mechanical processing of food?

The liver is primarily involved in the chemical processing of food rather than the mechanical breakdown of food. Its main functions include the production of bile, which aids in the digestion and absorption of fats, as well as the metabolism of nutrients absorbed from the digestive tract. While it plays a crucial role in the digestive system, it does not participate in the physical manipulation or grinding of food.

In contrast, the spleen is involved in filtering blood and recycling iron, and does not play a role in food processing at all. The kidneys are responsible for filtering blood and producing urine, and similarly do not process food. The gallbladder stores bile, which aids in fat digestion, but does not mechanically process food either. However, it is important to note that the other options (spleen, kidneys, and gallbladder) do not directly engage in the mechanical processing of food, but the liver is primarily focused on chemical processes. This distinction emphasizes why liver is considered the choice relating to the lack of mechanical processing.