In the context of ISS, which structure is categorized as a solid organ?

The spleen is categorized as a solid organ due to its specific physical and functional characteristics. Solid organs are typically defined by their dense, cohesive tissue structure, which allows them to perform complex functions. The spleen, in particular, plays a key role in the immune system and in the filtration of blood, serving functions such as recycling iron from red blood cells and producing lymphocytes.

When considering the other structures listed, the spinal cord is a part of the central nervous system and is primarily composed of nervous tissue, making it classified as a nervous tissue structure rather than a solid organ. The sternum, while it is a bone that provides structural support for the chest, is classified as a skeletal structure and not a solid organ in the same context as the spleen. The femur, being a long bone, also does not fit the classification of a solid organ as it serves primarily structural and functional roles related to movement and weight-bearing in the body.

Therefore, the spleen stands out as the only solid organ among the given options, highlighting its distinct anatomical and functional classification within the immune and circulatory systems.