Erythromycin is commonly used to treat which of the following?

Erythromycin is an antibiotic that is primarily used to treat bacterial infections. Its effectiveness in combating a range of infections makes it a common choice in medical practice. Specifically, erythromycin can target respiratory tract infections, skin infections, and certain types of acne, as it has been shown to reduce the bacteria that contribute to acne formation and inflammation. This dual role allows it to address both active infections and the acne associated with bacterial proliferation.

The other options don't align with erythromycin's pharmacological profile. For example, allergies typically require antihistamines or other allergy-specific medications rather than antibiotics like erythromycin. Similarly, dry skin is usually treated with moisturizers and topical agents rather than antibiotics, and hair loss can involve treatments like minoxidil or finasteride, which do not include antibiotic use. Therefore, option A is the best choice as it directly relates to erythromycin’s uses in treating infections and its application in dermatological conditions like acne.