Reactive Lymphadenopathy: This is the most likely diagnosis due to the common occurrence of reactive lymph node enlargement in response to infections, inflammation, or other benign conditions. The fact that the lymph nodes are described as "could be reactive" already points towards this possibility.

Infectious Mononucleosis: Caused by Epstein-Barr virus, this condition often presents with lymphadenopathy, particularly in the cervical region, along with fever and sore throat.
Tuberculosis: Although less common in some regions, TB can cause lymph node enlargement, especially in the neck, and is often accompanied by systemic symptoms like weight loss and night sweats.
Toxoplasmosis: A parasitic infection that can cause lymphadenopathy, usually in the absence of other significant symptoms.

Lymphoma: Both Hodgkin and non-Hodgkin lymphoma can present with lymph node enlargement. It's crucial to consider this diagnosis due to its significant implications for treatment and prognosis.
Metastatic Cancer: Cancers from various primary sites (e.g., head and neck, lung, breast) can metastasize to the lymph nodes in the neck, making this a critical diagnosis not to miss.
HIV Infection: Acute HIV infection can cause lymphadenopathy, and identifying this condition early is vital for the management and prevention of complications.

Castleman Disease: A rare disorder that involves an overgrowth of cells in the lymph nodes, which can cause lymphadenopathy.
Kikuchi-Fujimoto Disease: A rare, self-limiting condition that causes lymphadenopathy, typically in the neck, and is more common in young women.
Sarcoidosis: Although not exclusively rare, when it presents with lymphadenopathy in the neck, it can be a diagnostic challenge. Sarcoidosis is a condition characterized by the growth of granulomas in various parts of the body.