Final answer:
For a patient with a penicillin allergy and mononucleosis, erythromycin is often used as a second-line drug. Clarithromycin or azithromycin are alternative macrolides that could also be considered, tailoring the choice to the patient's specific clinical situation.
Step-by-step explanation:
When someone with a penicillin (PCN) allergy has mononucleosis (commonly referred to as 'mono') and requires treatment, healthcare providers typically consider a different class of antibiotics known as macrolides. The antibiotic erythromycin can be used as a second-line treatment. Alternatively, if warranted by the clinical scenario, antibiotics such as clarithromycin or azithromycin may be used, although careful consideration is essential because the use of certain antibiotics like ampicillin or amoxicillin in patients with mono and a PCN allergy may lead to a rash.
Thus, the final option to this question would involve assessing individual patient history, the severity of the allergy, and the necessity of antibiotic treatment.