Infectious Mononucleosis (Mono): This is a common condition in adolescents, caused by the Epstein-Barr virus, which can lead to lymphadenopathy, including enlarged lymph nodes on the chest wall. The age of the patient and the location of the lymph node make this a plausible diagnosis.

Tuberculosis (TB): Although less common in some regions, TB is a significant cause of lymphadenopathy worldwide and can affect the chest wall. It's more likely in areas with high TB prevalence or in individuals with exposure history.
Cat Scratch Disease: Caused by Bartonella henselae, this condition often results from a cat scratch or bite and can lead to lymphadenopathy, typically near the site of the scratch. Given the common interaction between adolescents and pets, this is a reasonable consideration.
Lymphoma: While less common, lymphomas such as Hodgkin lymphoma can present with lymphadenopathy in young individuals. The chest wall location could be involved, especially in the context of systemic disease.

Lymphoma (Hodgkin or Non-Hodgkin): Although mentioned earlier, it's crucial to reiterate the importance of not missing a diagnosis of lymphoma due to its significant implications for treatment and prognosis.
Metastatic Disease: In rare cases, an enlarged lymph node on the chest wall could be a sign of metastasis from another primary cancer. This would be particularly concerning in the context of known primary malignancies.
Sarcoidosis: This autoimmune condition can cause lymphadenopathy among other symptoms. While less common, missing sarcoidosis could lead to delayed treatment and potential complications.

Kikuchi-Fujimoto Disease: A rare, self-limiting condition that causes lymphadenopathy, typically in the neck but could potentially involve the chest wall. It's more common in young women.
Castleman Disease: A rare disorder that involves an overgrowth of cells in the lymphatic system, which can cause lymphadenopathy. It's unusual but could be considered in the differential diagnosis of unexplained lymph node enlargement.
Kimura Disease: Another rare condition characterized by lymphadenopathy and eosinophilia, more commonly seen in Asian males. It's an important consideration in the appropriate demographic and clinical context.