Syphilis (Primary): This is the most likely diagnosis due to the classic presentation of a painless, solitary ulcer (chancre) in the groin area, which is a common site for syphilis lesions. The chancre typically appears 3-4 weeks after exposure and heals spontaneously within 4-6 weeks.

Genital Herpes: Characterized by painful, multiple, shallow ulcers, which can appear in the groin area. The presence of systemic symptoms like fever and malaise can accompany the initial outbreak.
Chancroid: Caused by Haemophilus ducreyi, it presents with one or more painful ulcers and tender lymphadenopathy in the groin. The ulcers are typically deeper and more painful than those of syphilis.
Lymphogranuloma Venereum (LGV): An STD caused by certain strains of Chlamydia trachomatis, which can present with a small, painless lesion that may go unnoticed, followed by significant lymphadenopathy and potential genital ulcers.

HIV: Although not typically presenting with an open ulcer wound, HIV infection can increase susceptibility to other STDs that cause ulcers. Early diagnosis of HIV is crucial for treatment and prevention of complications.
Squamous Cell Carcinoma: In rare cases, an open ulcer in the groin could be a sign of a malignancy, especially in immunocompromised patients or those with a history of HPV infection.

Granuloma Inguinale (Donovanosis): A rare bacterial infection caused by Klebsiella granulomatis, characterized by painless, beefy-red ulcers without lymphadenopathy. It's more common in tropical and subtropical regions.
Mycoplasma genitalium: While more commonly associated with urethritis and cervicitis, Mycoplasma genitalium can cause ulcers, though this is less common and typically not the primary presentation.