Final answer:
The most likely diagnosis based upon the appearance of the genital lesions is genital herpes caused by HSV-2.
Step-by-step explanation:
Based on the appearance of the genital lesions described, the most likely diagnosis is genital herpes caused by HSV-2. Genital herpes is an STI caused by the herpes simplex virus and is characterized by painful blisters on the genitals. The virus remains in the body, causing periodic outbreaks of blisters throughout life. It is important to note that the lesions can occur on areas other than the genitals, so condom use may not always prevent transmission.