Which type of comedo is known as a whitehead, where the hair follicle is closed and sebaceous secretions remain trapped?

A closed comedo is commonly referred to as a whitehead. In this condition, the hair follicle is obstructed and is not open to the surface of the skin. Consequently, the sebaceous secretions, which include sebum and dead skin cells, become trapped beneath the skin's surface, leading to the characteristic white or flesh-colored appearance.

In contrast, open comedos, or blackheads, have a follicle that is open to the air, allowing the material to oxidize, which gives it a black or brown color. Subcutaneous comedos are not a standard term used in dermatology, and while acne vulgaris is a broader term that encompasses closed comedos, open comedos, and other inflamed lesions related to acne, it does not specifically refer to the closed type. Thus, identifying closed comedos as whiteheads accurately describes the condition defined in the question.