Angular Cheilitis: This condition is characterized by inflammation of the corners of the mouth, which can cause redness, swelling, and cracking. It is a common condition in adolescents and can be caused by a variety of factors, including vitamin deficiencies, allergies, or infections.

Cold Sore (Herpes Simplex Virus): Cold sores can cause redness, swelling, and blistering around the mouth, including the lower lips. They are highly contagious and common in adolescents.
Perioral Dermatitis: This is a skin condition that causes a rash around the mouth, which can include redness, itching, and small bumps. It is more common in females and can be triggered by various factors, including makeup, sunscreen, or certain medications.
Contact Dermatitis: This is a skin reaction that occurs after contact with an allergen or irritant, which can cause redness, itching, and swelling around the lower lips.

Lupus: Systemic lupus erythematosus (SLE) is an autoimmune disease that can cause a variety of symptoms, including skin rashes, which can appear around the mouth. Although rare in adolescents, it is essential to consider this diagnosis due to its potential severity.
Sarcoidosis: This is a rare autoimmune disease that can cause skin lesions, including around the mouth. It can also affect other organs, such as the lungs and eyes.

Staphylococcal Infection: A bacterial infection caused by Staphylococcus aureus can cause redness, swelling, and pus around the lower lips.
Histoplasmosis: A rare fungal infection that can cause skin lesions, including around the mouth, although it is more commonly associated with lung symptoms.
Gastroesophageal Reflux Disease (GERD): Although not typically associated with skin symptoms, severe GERD can cause irritation and redness around the mouth due to acid reflux.