Tonsillitis: This is the most likely diagnosis given the symptoms of an injected throat with enlarged tonsils. Tonsillitis is an inflammation of the tonsils, which can be caused by viral or bacterial infections, and it typically presents with symptoms such as sore throat, fever, and enlarged tonsils.

Pharyngitis: Although the question specifically mentions an enlarged tonsil, pharyngitis (inflammation of the pharynx) can sometimes present with similar symptoms, including a sore throat and fever. It's possible for pharyngitis and tonsillitis to coexist.
Mononucleosis: This viral infection, caused by the Epstein-Barr virus, can lead to symptoms that include a sore throat, fever, and significantly enlarged tonsils. It's a common condition in adolescents and young adults.

Peritonsillar Abscess: This is a serious complication of tonsillitis where an abscess forms beside the tonsils. It's characterized by severe throat pain, difficulty swallowing, and a deviated uvula. Although less common, it's a medical emergency that requires prompt treatment.
Lemierre's Syndrome: A rare but potentially life-threatening condition that starts as a throat infection (often pharyngitis or tonsillitis) and progresses to a bloodstream infection, leading to septicemia and other complications.
Epiglottitis: An inflammation of the epiglottis that can be life-threatening due to the risk of airway obstruction. It presents with severe sore throat, difficulty swallowing, and shortness of breath.

Tonsillar Cancer: Although extremely rare, especially in younger populations, cancer of the tonsils can present with an enlarged tonsil. It's more common in adults and is often associated with a history of smoking or alcohol use.
Granulomatosis with Polyangiitis (formerly known as Wegener's Granulomatosis): A rare autoimmune disorder that can affect various parts of the body, including the throat, and may present with symptoms similar to tonsillitis or pharyngitis, among other systemic symptoms.