Streptococcal Pharyngitis (Strep Throat): This is a common cause of sore throat in adolescents, and although it typically presents acutely, some cases can have a prolonged course. The prolonged duration in this case could be due to incomplete treatment or a recurrent infection.

Viral Pharyngitis: Many viruses can cause pharyngitis, and some, like mononucleosis (caused by Epstein-Barr virus), can lead to prolonged symptoms.
Tonsillitis: Either bacterial or viral, tonsillitis can cause chronic or recurrent sore throat, especially if the tonsils are enlarged or have a history of frequent infections.
Postnasal Drip (Upper Airway Secretion): Chronic rhinitis or sinusitis can lead to postnasal drip, irritating the throat and causing a sore throat that lasts for weeks.

Lymphoma: Although rare, lymphomas can present with prolonged sore throat due to lymph node enlargement in the neck. It's crucial to consider this in the differential due to its severe implications.
HIV Infection: Initial infection with HIV can cause a variety of symptoms, including a prolonged sore throat. Given the potential long-term consequences, missing this diagnosis could be significant.
Lemierre's Syndrome: A rare but serious condition that starts with a sore throat and can progress to include neck pain and fever, due to a bacterial infection (usually Fusobacterium necrophorum) that spreads to the jugular vein.

Gonococcal Pharyngitis: Although more commonly associated with genital infections, Neisseria gonorrhoeae can cause pharyngitis, especially in sexually active adolescents.
Tuberculous Pharyngitis: A rare form of tuberculosis that affects the pharynx, more common in areas with high TB prevalence or in immunocompromised individuals.
Sarcoidosis: A systemic disease that can rarely cause chronic sore throat among other symptoms, due to granulomatous inflammation in various parts of the body, including the lymph nodes in the neck.