A couple, who did not know each other very well, dated and had sexual contact. Several weeks later, the man noticed a small, painless vesicle on his penis, which ruptured and then healed. Soon, his inguinal lymph nodes enlarged and discharged pus through multiple sinus tracts. Lymphogranuloma venereum (LGV) is a venereal disease caused by serotype L1, L2, or L3 of Chlamydia trachomatis. The differential diagnosis should include which of the following?

The differential diagnosis of LGV includes syphilis, genital herpes, and chancroid. Several clinical tests can be used to rule out syphilis and genital herpes. These include a negative (negative to rule out; positive to rule in) dark-field examination as well as positive sero-logic findings for syphilis and the demonstration of herpes simplex virus by cytology or culture. Haemophilus ducreyi can usually be isolated from the ulcer in chancroid. Babesiosis is an RBC-infecting tick-borne disease, usually derived from dogs or Texas cattle. Most cases are asymptomatic, but clindamycin or azithromycin are effective treatments. Mononucleosis is seen most often in infectious mononucleosis caused by EBV. Psittacosis is caused by inhaling aerosols or direct contact with birds infected with C. psittaci and is usually a mild upper respiratory illness. Shingles would be evidenced by masses of painful herpetic lesions caused by reactivation of VZV (chicken pox).