Infectious lymphadenitis: This is the most common cause of lymphadenopathy in children, often resulting from a viral or bacterial infection. The inguinal location suggests a possible infection of the lower extremities or genital area.

Cat-scratch disease: Caused by Bartonella henselae, this condition often presents with lymphadenopathy, typically in the axillary or neck region, but can also occur in the inguinal area if the scratch is on the leg.
Tuberculosis: Although less common in some regions, tuberculosis can cause lymphadenitis, especially in areas where TB is prevalent. The inguinal location is less typical but possible.
Local infection or abscess: An infection or abscess in the vicinity of the inguinal lymph node, such as a skin infection on the leg or a perianal abscess, can cause reactive lymphadenopathy.

Lymphoma: Although rare in children, lymphoma can present with lymphadenopathy. It is crucial to consider this diagnosis to ensure timely treatment if present.
HIV infection: Children with HIV can present with persistent generalized lymphadenopathy, and it's essential to consider this in the differential diagnosis, especially if there are other systemic symptoms or risk factors.
Malignancy (e.g., neuroblastoma, rhabdomyosarcoma): Certain malignancies can metastasize to lymph nodes, and while rare, missing such a diagnosis could have severe consequences.

Kikuchi-Fujimoto disease: A rare form of lymphadenitis that is more common in Asian females but can occur in any demographic. It typically presents with cervical lymphadenopathy but can occur in other locations.
Castleman disease: A rare disorder that can cause lymphadenopathy, often presenting with systemic symptoms. It is more commonly seen in adults but can occur in children.
Sarcoidosis: Although rare in children, sarcoidosis can cause lymphadenopathy and should be considered, especially if there are other systemic symptoms or findings suggestive of the disease.