Which vitamin is crucial for vision and immune function?

Vitamin A is essential for both vision and immune function due to its role in various physiological processes. It is a key component of rhodopsin, a pigment found in the retina that is necessary for the formation of images in low-light conditions, thereby supporting night vision. Moreover, vitamin A contributes to the maintenance of healthy epithelial tissues, including the skin and mucous membranes, which are crucial for the body's first line of defense against infections. It also plays a pivotal role in the regulation of the immune system, enhancing the activity of various immune cells, thereby helping the body to fend off illnesses.

The other vitamins listed have important health benefits but do not play the same vital roles in vision and immune function that vitamin A does. For instance, vitamin K is primarily involved in blood clotting, vitamin C plays a significant role in antioxidant protection and collagen synthesis, and vitamin E acts as an antioxidant but does not have a direct effect on vision or the immune system in the same way as vitamin A.