Which hair cycle phase involves hair follicles undergoing regression?

The phase of the hair cycle that involves hair follicles undergoing regression is known as catagen. During this transitional phase, which follows the anagen phase (the active growth phase), the hair follicle begins to shrink, leading to a decrease in the size of the hair. This process involves the detachment of the hair from the follicle and a significant reduction in metabolic activity within the follicle itself. The catagen phase typically lasts for a few weeks, during which the hair strand becomes dormant and is eventually shed.

Anagen is characterized by active hair growth, while telogen is the resting phase in which the hair is not growing but is still anchored in the follicle. Progenitor is not a recognized phase in the hair cycle. Therefore, catagen is uniquely identified as the phase where follicles regress, making it the correct choice.