For a 16 year-old female with worsening acne, which of the following treatments would be appropriate to add?

Oral doxycycline is an appropriate treatment option for a 16-year-old female with worsening acne, primarily because it is an effective systemic antibiotic used to combat inflammatory acne. By targeting the bacteria Propionibacterium acnes, doxycycline helps reduce inflammation, which can significantly improve acne lesions and prevent new ones from forming.

Antibiotics like doxycycline are particularly effective for moderate to severe acne cases, where topical treatments alone may not yield satisfactory results. This oral antibiotic not only decreases the bacterial load but also possesses anti-inflammatory properties, making it a suitable choice in cases where the acne is becoming increasingly problematic.

Other treatments listed might not be as effective for this patient's specific situation. For example, intralesional steroids are usually reserved for isolated, inflammatory nodules or cysts rather than for widespread acne. Benzoyl peroxide gel is a good topical option, particularly in mild to moderate cases, but it may not be sufficient alone for worsening acne. Metronidazole cream is more commonly indicated for rosacea or inflammatory acne but is not the first-line treatment for teenage acne. Therefore, oral doxycycline emerges as a more comprehensive solution for addressing the worsening acne in this scenario.