Reactive Lymphadenitis: This is the most common cause of lymphadenopathy in the general population, often resulting from a viral upper respiratory infection. The lymph nodes at the back of the neck are frequently involved due to their location, which makes them prone to react to infections in the head and neck area.

Tuberculosis (TB): TB can cause lymphadenopathy, particularly in the neck, and is more common in individuals who have been exposed to TB or have a weakened immune system. The diagnosis should be considered, especially if the patient has risk factors such as travel to or residence in areas with high TB prevalence.
Lymphoma: Both Hodgkin and non-Hodgkin lymphoma can present with lymphadenopathy. The back of the neck is a common site for lymph node involvement, and lymphoma should be considered, especially in older adults or those with systemic symptoms like weight loss or fever.
Metastatic Cancer: Cancers from the head and neck region, such as squamous cell carcinoma, can metastasize to lymph nodes in the neck, including those at the back of the neck. A history of cancer or symptoms suggestive of a primary tumor should prompt consideration of this diagnosis.

Hodgkin Lymphoma: Although included in the "Other Likely Diagnoses" category, it's crucial to emphasize that missing a diagnosis of Hodgkin lymphoma could have significant implications for treatment and prognosis. It often presents with lymphadenopathy, and the back of the neck can be involved.
Infectious Mononucleosis (Mono): Caused by Epstein-Barr virus (EBV), infectious mononucleosis can lead to significant lymphadenopathy, including in the posterior cervical region. It's essential to consider this diagnosis, especially in young adults, due to its potential to cause splenomegaly and the risk of splenic rupture.
Human Immunodeficiency Virus (HIV): Acute HIV infection can cause lymphadenopathy, and it's critical not to miss this diagnosis due to its significant implications for long-term health and the importance of early antiretroviral therapy.

Kikuchi-Fujimoto Disease: A rare, self-limiting condition that predominantly affects young women, characterized by lymphadenopathy, often in the neck, and fever. It's essential to consider this diagnosis to avoid unnecessary treatments.
Castleman Disease: A rare disorder that involves an overgrowth of cells in the lymphatic system, which can cause lymphadenopathy. It's crucial to diagnose Castleman disease due to its potential for complications and the need for specific treatment.
Sarcoidosis: Although more commonly associated with pulmonary symptoms, sarcoidosis can cause lymphadenopathy, including in the neck. It's a diagnosis to consider, especially if other systemic symptoms or pulmonary findings are present.