Which of the following is NOT a hollow organ?

The spleen is the correct choice as the organ that is not classified as a hollow organ. Hollow organs are typically those that have a cavity and are involved in storage or the passage of substances, such as food or fluids. The small intestine, bladder, and gallbladder all have luminal spaces and actively participate in the digestion or storage of substances.

The small intestine is a segment of the gastrointestinal tract, designed to digest and absorb nutrients from food; it has a hollow lumen through which food passes. The bladder is a muscular sac that stores urine until it is expelled from the body, so it also qualifies as a hollow organ. Lastly, the gallbladder is responsible for storing bile produced by the liver and has a cavity that holds this fluid. In contrast, the spleen is a solid organ with no central cavity, primarily involved in filtering blood and immune function rather than storing or transporting materials.