Infectious Mononucleosis (IMN) caused by Epstein-Barr Virus (EBV): The presence of both high EBV IgG and elevated EBV IgM antibodies strongly suggests a current or recent EBV infection. IMN typically presents with fever, sore throat, and lymphadenopathy, but the absence of remembered fever or sore throat does not rule out the diagnosis, as symptoms can be mild or forgotten.

Streptococcal Pharyngitis or Tonsillitis: Although the patient does not recall a sore throat, streptococcal infections can cause lymphadenopathy and neck pain. A throat culture or rapid streptococcal antigen test could help diagnose this condition.
Viral Pharyngitis: Other viruses can cause similar symptoms, including lymphadenopathy and neck pain, without a significant elevation in EBV antibodies. However, the specific viral etiology might be harder to determine without further testing.
Toxoplasmosis: This parasitic infection can cause lymphadenopathy, particularly in the cervical region, but it is less likely given the EBV serology results.

Lymphoma: Although rare in children, lymphoma can present with lymphadenopathy and must be considered, especially if the lymph node does not resolve with conservative management. Further imaging and potentially a biopsy might be necessary.
Tuberculosis (TB): TB can cause cervical lymphadenopathy and is a critical diagnosis not to miss due to its public health implications and the need for specific treatment. A purified protein derivative (PPD) test or interferon-gamma release assay (IGRA) could help diagnose TB.
Hodgkin Lymphoma: This type of cancer can also present with lymphadenopathy, often in the cervical region, and is crucial to diagnose early for effective treatment.

Cat-Scratch Disease: Caused by Bartonella henselae, this infection can result in lymphadenopathy, typically after a cat scratch. It's less likely given the lack of reported exposure but remains a possibility.
Kawasaki Disease: Although it primarily affects younger children, Kawasaki disease can cause lymphadenopathy and neck pain. However, it is typically accompanied by other symptoms like fever, rash, and conjunctivitis.
Brucellosis: A zoonotic infection that can cause lymphadenopathy among other symptoms. It's rare and usually associated with specific exposures (e.g., to infected animals or their products).