What is the most likely diagnosis for a patient presenting with painless cervical lymphadenopathy, fever, weight loss, and night sweats accompanied by mediastinal mass and Reed-Sternberg cells?

The symptoms of painless cervical lymphadenopathy, fever, weight loss, and night sweats are classic presentations that suggest a lymphoproliferative disorder. The presence of a mediastinal mass further supports this diagnosis, especially in pediatric patients.

Reed-Sternberg cells are unique to Hodgkin's Lymphoma and serve as a key diagnostic criterion for this condition. These atypical giant cells are derived from B lymphocytes and are found in lymph nodes affected by Hodgkin's Lymphoma, contributing to the characteristic clinical features of the disease such as those presented by the patient. The combination of symptoms and the histological finding of Reed-Sternberg cells unequivocally points toward Hodgkin's Lymphoma, making it the most likely diagnosis.

In contrast, Non-Hodgkin's Lymphoma generally presents differently and does not have Reed-Sternberg cells. Acute Lymphoblastic Leukemia (ALL) and Chronic Lymphocytic Leukemia (CLL) also have distinct clinical and laboratory features that would not explain the specific combination of lymphadenopathy, fever, and mediastinal mass in the same way, particularly in the context of Reed-Sternberg cells. Thus, the presentation aligns perfectly with