Infectious Mononucleosis (IM): Caused by Epstein-Barr virus (EBV), this diagnosis is the most likely due to the presentation of typical symptoms such as fever, sore throat, lymphadenopathy, and fatigue, especially in adolescents and young adults.

Streptococcal Pharyngitis: Presents with sore throat, fever, and lymphadenopathy, similar to IM, but is caused by Group A beta-hemolytic streptococcus.
Cytomegalovirus (CMV) Infection: Can mimic IM in its presentation, including fever, fatigue, and lymphadenopathy, particularly in immunocompromised individuals.
Toxoplasmosis: Although less common, can present with similar symptoms to IM, especially in individuals with weakened immune systems.

Hodgkin Lymphoma: A type of cancer that can present with lymphadenopathy, fever, and fatigue, making it crucial not to miss, despite its lower likelihood.
Acute Lymphoblastic Leukemia (ALL): Presents with symptoms that can overlap with IM, such as fatigue, lymphadenopathy, and fever, and is critical to diagnose early.
AIDS (Acquired Immunodeficiency Syndrome): The initial presentation of AIDS can sometimes mimic IM, with symptoms like fever, fatigue, and lymphadenopathy, making it a diagnosis that should not be overlooked.

Human Immunodeficiency Virus (HIV) Acute Retroviral Syndrome: Presents with symptoms similar to IM, including fever, rash, and lymphadenopathy, but is less common and requires specific testing for diagnosis.
Infectious Diseases like Tuberculosis or Brucellosis: Although rare in the context of IM-like symptoms, these diseases can present with similar clinical pictures and are important to consider, especially in endemic areas or in individuals with specific risk factors.
Autoimmune Lymphoproliferative Syndrome (ALPS): A rare genetic disorder that can mimic IM with chronic lymphadenopathy, splenomegaly, and autoimmune cytopenias.