What is the primary function of vitamin A?

Vitamin A plays a crucial role in maintaining healthy vision, particularly in low-light conditions. It is a key component of rhodopsin, a protein in the eyes that allows us to see in dim light. Additionally, vitamin A is vital for the proper functioning of the immune system; it helps maintain the integrity of skin and mucosal cells, which are the body's first lines of defense against pathogens. This combination of supporting vision and enhancing immune function underscores the primary role of vitamin A in the body.

Other options, such as aiding in digestion, synthesizing proteins, or regulating blood sugar levels, are important functions of various nutrients and macronutrients but do not directly relate to the primary functions attributed to vitamin A. Thus, the focus on vision and immune support highlights why this choice is the most accurate.