Infectious Mononucleosis: This condition, caused by Epstein-Barr virus, often presents with lymphadenopathy (enlarged lymph nodes) in multiple regions, including the groin and abdomen, along with fever, sore throat, and fatigue. Its prevalence and the common presentation make it a leading consideration.

Lymphoma: Both Hodgkin's and non-Hodgkin's lymphoma can cause lymphadenopathy in the groin and abdomen. The presence of systemic symptoms such as weight loss, fever, and night sweats would increase the likelihood of this diagnosis.
Tuberculosis: Although less common in some regions, tuberculosis (TB) can cause lymphadenopathy, particularly in the abdomen. A history of exposure, travel, or residence in areas with high TB prevalence, along with systemic symptoms like weight loss and cough, would support this diagnosis.
Metastatic Cancer: Cancers such as melanoma, testicular cancer, or gastrointestinal cancers can metastasize to lymph nodes in the groin and abdomen. A history of cancer or findings suggestive of a primary tumor would make this diagnosis more likely.

HIV Infection: Early HIV infection can present with lymphadenopathy, among other symptoms. Missing this diagnosis could lead to delayed treatment and significant morbidity.
Syphilis: This sexually transmitted infection can cause lymphadenopathy, particularly in the groin. It's crucial to consider syphilis due to its potential for serious complications if left untreated.
Cat-Scratch Disease: Caused by Bartonella henselae, this infection can result in lymphadenopathy, typically near the site of the cat scratch. Although usually self-limiting, it can be severe in immunocompromised individuals.

Castleman Disease: A rare disorder that involves an overgrowth of cells in the lymph nodes, which can cause lymphadenopathy. It can be unicentric (localized) or multicentric (widespread), with the latter often associated with more severe symptoms and potential for malignancy.
Kikuchi-Fujimoto Disease: A rare, self-limiting condition that causes lymphadenopathy, typically in the neck but can be generalized. It's more common in young women and of Asian descent.
Whipple's Disease: A rare, systemic bacterial infection (Tropheryma whipplei) that can cause lymphadenopathy among other symptoms like diarrhea, weight loss, and arthralgias. It's crucial to consider due to its potential for severe complications if not treated.