Differential Diagnosis for Red Bumps on the Head of Penis and Itching after Sexual Intercourse

Single Most Likely Diagnosis

• Genital Herpes: This is a common sexually transmitted infection (STI) that can cause red bumps, blisters, or sores on the genitals, along with itching, burning, or pain. The symptoms appearing 3 weeks after sexual intercourse fit within the typical incubation period of genital herpes.

Other Likely Diagnoses

• Syphilis: Another STI that can cause sores or bumps on the genitals. The primary stage of syphilis is characterized by a painless sore, but it can be accompanied by itching in some cases.
• Genital Warts (Human Papillomavirus, HPV): These can appear as red bumps or growths on the genitals and are known to cause itching. They are a common STI and can develop weeks to months after exposure.
• Contact Dermatitis: An allergic reaction or irritation from condoms, lubricants, or other products used during sexual intercourse could lead to red bumps and itching.

Do Not Miss Diagnoses

• Primary HIV Infection: Although less common, primary HIV infection can present with a rash, which might include the genital area, along with flu-like symptoms. Given the potential severity of missing an HIV diagnosis, it's crucial to consider this in the differential.
• Lymphogranuloma Venereum (LGV): A type of STI caused by certain strains of Chlamydia trachomatis, which can lead to genital ulcers, lymphadenopathy, and other systemic symptoms. It's essential to diagnose LGV promptly due to its potential for serious complications.

Rare Diagnoses

• Molluscum Contagiosum: A viral infection that causes small, pearly, or flesh-colored bumps on the skin. While it can appear on the genitals, it's less commonly associated with itching and is more often seen in children or immunocompromised individuals.
• Scabies: Caused by a mite, scabies can lead to severe itching and a pimple-like skin rash. It's less common on the genitals but can occur, especially in the context of sexual transmission.

Each of these diagnoses has different treatment approaches, ranging from antiviral medications for herpes and HIV, antibiotics for syphilis and LGV, to topical treatments for genital warts and contact dermatitis. Accurate diagnosis through clinical evaluation, laboratory tests (such as PCR for herpes, serology for syphilis, and HIV testing), and sometimes biopsy is essential for guiding appropriate treatment.