Infectious Mononucleosis: This is a common condition in adolescents, caused by Epstein-Barr virus (EBV), which often presents with fever, sore throat, and lymphadenopathy. The age of the patient and the nonspecific nature of the symptoms make this a plausible initial consideration.

Viral Upper Respiratory Tract Infections: These are common in adolescents and can cause prolonged fever. The diagnosis might be challenging if the typical symptoms of a cold or flu are not prominent.
Streptococcal Pharyngitis: Although it typically presents with a sore throat, streptococcal infections can sometimes cause fever without significant throat symptoms, especially in the early stages.
Influenza: Seasonal influenza can cause fever of unknown origin, especially during outbreaks. The absence of respiratory symptoms does not rule out influenza, as some patients may present atypically.

Meningitis or Encephalitis: These are life-threatening conditions that require immediate diagnosis and treatment. Early symptoms can be nonspecific, including fever, headache, and malaise, before more severe neurological signs develop.
Septicemia or Bacteremia: Bacterial infections in the bloodstream can present with fever as the only initial symptom. Prompt identification and treatment are crucial to prevent severe complications or death.
Lymphoma or Leukemia: Although rare, these malignancies can present with prolonged fever of unknown origin. A high index of suspicion is necessary, especially if there are other signs such as weight loss, night sweats, or lymphadenopathy.
Tuberculosis (TB): TB can cause fever of unknown origin, especially in endemic areas or in individuals with risk factors for exposure. It's crucial to consider TB due to its public health implications and the need for specific treatment.

Autoimmune Disorders (e.g., Systemic Lupus Erythematosus, Juvenile Idiopathic Arthritis): These conditions can present with fever and nonspecific symptoms. They are less common but should be considered if initial investigations for infectious causes are negative.
Endocarditis: Infective endocarditis is rare in healthy adolescents without heart disease but can occur. It's a critical diagnosis to consider due to its high morbidity and mortality if untreated.
Brucellosis: This zoonotic infection is rare in many areas but can cause fever of unknown origin. It should be considered in individuals with exposure to animals or unpasteurized dairy products.
Cat-Scratch Disease: Caused by Bartonella henselae, this infection can lead to prolonged fever, especially if there's a history of cat scratches or bites.