What is the most likely medication causing a rash in a 21-year-old being treated for acne?

In this scenario, the medication most likely causing a rash in a 21-year-old being treated for acne is Minocycline. This medication is a type of antibiotic commonly used for acne treatment and is well-known to cause skin reactions, including rashes, in some patients.

Minocycline can lead to hypersensitivity reactions, which manifest as rash, especially within the first few weeks of treatment. The risk is heightened in younger individuals, making it a more probable cause of the rash compared to the other medications listed.

Oral Lymecycline, although it can also cause skin reactions, is less commonly associated with significant rashes than Minocycline. Topical Benzoyl Peroxide typically irritates the skin but is not commonly linked to systemic rash reactions. Oral Isotretinoin, while it has significant side effects, including skin dryness, does not typically cause a rash as a common side effect compared to Minocycline.

Thus, when considering the likelihood of a rash as a side effect, Minocycline stands out as the most probable cause.