Metastatic malignancy: Given the size (19mm) and location (femoral region) of the lymph node, a metastatic malignancy from a primary site such as the lower limb, pelvic area, or even from more distant sites like the breast, lung, or gastrointestinal tract, is highly plausible. The hardness of the nodule further supports this possibility.

Lymphoma: Both Hodgkin's and non-Hodgkin's lymphoma can present with lymphadenopathy, and the femoral region is a common site. The hardness of the node could be indicative of lymphomatous involvement.
Infectious lymphadenitis: Chronic infections such as tuberculosis or fungal infections can cause hard lymph nodes, although they are less common in the femoral region compared to other areas like the neck or axilla.
Sarcoidosis: This systemic disease can cause lymphadenopathy, including in the femoral region, although it is less common. The lymph nodes in sarcoidosis can be hard and non-tender.

Melanoma metastasis: Given the location in the femoral region, which drains the lower limb, a metastasis from a melanoma, even if the primary site is not immediately apparent, must be considered due to its aggressive nature and potential for late presentation.
Testicular cancer metastasis: Although less common, testicular cancer can metastasize to the lymph nodes in the femoral region, especially if the primary tumor is located in the lower abdomen or if there's retroperitoneal lymph node involvement.

Castleman disease: A rare disorder that involves an overgrowth of cells in the lymphatic system, which can present as lymphadenopathy, including in the femoral region.
Kikuchi-Fujimoto disease: A rare, self-limiting condition that causes lymphadenopathy, more commonly in the neck but can occur elsewhere, including potentially the femoral region.
Kimura disease: A chronic inflammatory disorder that can cause lymphadenopathy and eosinophilia, more common in Asian males, and typically involves the head and neck region but can rarely affect other areas.