Which of the following is NOT classified as a lymphoid organ?

The liver is not classified as a lymphoid organ. Instead, it is primarily an organ involved in metabolism, detoxification, and the production of biochemicals necessary for digestion, such as bile. While the liver does contain immune cells and plays a role in the body's immune response, it is not primarily associated with the lymphatic system or the production of lymphocytes.

In contrast, the thymus, tonsils, and spleen are all key components of the lymphatic system. The thymus is vital for the development and maturation of T-lymphocytes, which are crucial for adaptive immunity. The tonsils help filter pathogens that enter through the mouth and nose and contribute to immune surveillance. The spleen filters blood and helps manage the immune response by producing and storing lymphocytes. Each of these organs has a direct role in the immune system, making them classified as lymphoid organs, while the liver's primary functions lie outside this classification.