A 29-year-old man comes to the clinic because of progressive hair thinning over the past 4 years. He says his father and paternal grandfather developed similar hair loss in their 20s. He has no scalp pain, pruritus, or scaling. Physical examination shows symmetric thinning over the vertex and frontal scalp with preservation of the occipital hairline. The scalp skin appears normal without erythema or scarring. Which of the following is the most likely pathogenic mediator of this patient's condition?