What is the likely diagnosis for a 34-year-old male presenting with a 5-day history of purulent anal discharge, who engages in unprotected (condomless) receptive anal sex with multiple male partners in saunas, and had a negative (NAD) sexually transmitted infection (STI) test 3 months ago?

Differential Diagnosis for 34-year-old Male with Purulent Anal Discharge

Single Most Likely Diagnosis

• Gonococcal Proctitis: This is the most likely diagnosis given the patient's symptoms of purulent anal discharge and history of condomless receptive anal sex with a stranger. Neisseria gonorrhoeae is a common cause of sexually transmitted infections (STIs) that can lead to proctitis, characterized by anal discharge, pain, and bleeding.

Other Likely Diagnoses

• Chlamydial Proctitis: Similar to gonococcal proctitis, Chlamydia trachomatis can cause proctitis, leading to symptoms like anal discharge and pain. The patient's sexual history increases the risk for this infection.
• Herpes Simplex Virus (HSV) Proctitis: HSV can cause proctitis, especially in individuals with a history of receptive anal sex. Symptoms may include anal pain, discharge, and vesicles or ulcers.
• Syphilitic Proctitis: Caused by Treponema pallidum, syphilitic proctitis is less common but should be considered in the differential diagnosis, especially given the patient's high-risk sexual behavior.

Do Not Miss Diagnoses

• HIV Infection: Although not directly causing purulent anal discharge, HIV infection can significantly increase the susceptibility to other STIs and can present with various symptoms, including those similar to other STIs. Given the patient's high-risk sexual behavior, testing for HIV is crucial.
• Lymphogranuloma Venereum (LGV): Caused by certain serovars of Chlamydia trachomatis, LGV can lead to proctitis, especially in men who have sex with men (MSM). It's a critical diagnosis to consider due to its potential for severe complications if left untreated.

Rare Diagnoses

• Anal Tuberculosis: Although rare in many parts of the world, anal tuberculosis can cause symptoms similar to those of other proctitides, including discharge and pain. It should be considered in individuals with risk factors for tuberculosis.
• Amoebic Proctitis: Caused by Entamoeba histolytica, this condition can lead to anal discharge and pain. It's more common in travelers to or immigrants from endemic areas but can also be sexually transmitted, particularly among MSM.