Cat Scratch Disease: This is a common cause of unilateral lymphadenopathy, especially in young adults who have been exposed to cats. The lymphadenopathy is typically tender and can be accompanied by other symptoms such as fever and a papule or pustule at the site of the cat scratch.

Sexually Transmitted Infections (STIs): Given the location of the lymphadenopathy, STIs such as chlamydia, gonorrhea, or syphilis should be considered, especially if the patient is sexually active. These infections can cause unilateral or bilateral inguinal lymphadenopathy.
Viral Infections: Viruses like herpes simplex or cytomegalovirus can cause lymphadenopathy, including in the inguinal region. The presence of other symptoms such as fever, rash, or genital ulcers can help guide the diagnosis.
Bacterial Infections: Bacterial infections, including those caused by Staphylococcus aureus or Streptococcus pyogenes, can lead to lymphadenitis, which is an infection of the lymph nodes. This condition is typically accompanied by signs of infection such as redness, warmth, and tenderness.

Lymphoma: Although less common, lymphoma can present with lymphadenopathy. It is crucial to consider this diagnosis to ensure timely referral and treatment if necessary.
Tuberculosis: TB can cause lymphadenitis, and while it might not be the first consideration in all regions, it's a critical diagnosis not to miss due to its public health implications and the need for specific treatment.
HIV: Inguinal lymphadenopathy can be a presentation of acute HIV infection. Given the significant implications for treatment and prevention of transmission, HIV should be considered in the differential diagnosis.

Kikuchi-Fujimoto Disease: A rare form of lymphadenitis that typically affects young women, presenting with tender cervical lymphadenopathy but can also involve other lymph node groups.
Castleman Disease: A rare disorder that involves an overgrowth of cells in the lymphatic system, which can cause lymphadenopathy among other symptoms.
Malignant Melanoma or Other Cancers: Metastasis to inguinal lymph nodes from a primary malignancy, such as melanoma, is a rare but serious cause of unilateral lymphadenopathy.