Which of the following is NOT a function of the skin?

The skin has several important functions, including thermoregulation, pigmentation, and acting as a barrier. In this context, when considering the option of producing vitamin K, it is important to note that while the skin plays a crucial role in synthesizing vitamin D when exposed to sunlight, it is not involved in the production of vitamin K. Instead, vitamin K is primarily produced by bacteria in the gut and obtained through dietary sources such as green leafy vegetables.

Thermoregulation is achieved through mechanisms like sweating and adjusting blood flow to the skin. Pigmentation, provided by melanin, is important for protecting the skin from ultraviolet radiation. Pores on the skin serve various functions, such as allowing sweat and sebum to reach the surface, but they do not serve a purpose related to ventilation in the respiratory sense. Thus, the correct choice focuses on the fact that the skin does not produce vitamin K, making it distinct from the essential functions that the skin performs.