A 27-year-old woman presents to her primary care physician with a painful, burning sensation on her lower lip that started two days ago. She reports that the area initially felt tingly and itchy, but has since developed into small, fluid-filled blisters clustered near the corner of her mouth. The blisters are painful to touch and have begun to rupture, forming shallow ulcers.

She recalls experiencing similar episodes in the past, often triggered by stress, fatigue, or prolonged sun exposure. Her medical history is unremarkable, and she denies any recent fever or systemic symptoms. She has not been using any new skincare products and does not recall any allergic reactions.

On physical examination, a cluster of small vesicles on an erythematous base is noted on the right side of her lower lip. Some of the vesicles have ruptured, leaving crusted erosions. No lymphadenopathy is detected, and the rest of her exam is unremarkable.

Given the recurrent nature and classic appearance of the lesions, a diagnosis of herpes simplex virus (HSV-1) infection, also known as oral herpes or a cold sore, is made. The patient is offered antiviral treatment (acyclovir or valacyclovir) to shorten the duration of symptoms and is advised to avoid touching the lesions to prevent autoinoculation and transmission to others. She is also counseled on triggers and preventive measures, including the use of sunscreen on her lips and stress management.