Viral Pharyngitis: This is the most common cause of reactive lymphadenopathy in children, often resulting from viral infections such as adenovirus, enterovirus, or influenza. The location in the posterior neck and the reactive nature of the lymph node support this diagnosis.

Bacterial Pharyngitis: Caused by Group A beta-hemolytic streptococcus, this condition can also lead to reactive lymphadenopathy. The presentation might be similar to viral pharyngitis, but the presence of a sore throat and fever could hint towards this diagnosis.
Infectious Mononucleosis: Caused by Epstein-Barr virus (EBV), this condition often presents with lymphadenopathy, fever, and sore throat. Although it's more common in adolescents and young adults, it can occur in younger children.
Dental or Skin Infections: Localized infections in the head and neck region can cause reactive lymphadenopathy. The proximity of the lymph node to potential sources of infection (e.g., teeth, skin) makes these conditions plausible.

Lymphoma: Although less common, lymphoma (Hodgkin or non-Hodgkin) can present with lymphadenopathy. It's crucial to consider this diagnosis to ensure timely intervention if present.
Tuberculosis: TB can cause lymphadenitis, particularly in the neck. Given its potential severity and the need for specific treatment, it's essential not to miss this diagnosis, especially in areas with high TB prevalence or in individuals with risk factors.
Cat-Scratch Disease: Caused by Bartonella henselae, this condition can lead to lymphadenopathy, typically after a cat scratch or bite. It's more common in children and young adults.

Kawasaki Disease: An acute febrile illness that predominantly affects children, characterized by lymphadenopathy, rash, and other systemic symptoms. It's rare but important to consider due to its potential for cardiac complications.
Sarcoidosis: A systemic granulomatous disease that can cause lymphadenopathy, among other symptoms. It's rare in children and would be an unusual cause of a solitary reactive lymph node.
Castleman Disease: A rare lymphoproliferative disorder that can present with lymphadenopathy. It's more commonly seen in adults but can occur in children, making it a rare but potential diagnosis to consider.