Differential Diagnosis for Swollen Penis and Itchy Anus

Given the symptoms of a swollen penis and itchy anus with negative gonorrhea and chlamydia tests, the following differential diagnoses are considered:

• Single Most Likely Diagnosis
  • Eczema/Dermatitis: This is a common condition that can cause itching and swelling in the genital area, including the penis and around the anus. The negative tests for gonorrhea and chlamydia make an infectious cause less likely, pointing towards a dermatological condition.
• Other Likely Diagnoses
  • Psoriasis: A chronic skin condition that can cause itching, redness, and swelling. Genital psoriasis can manifest with similar symptoms.
  • Contact Dermatitis: An allergic reaction or irritation from something that has come into contact with the skin, such as soaps, lotions, or condoms, could cause these symptoms.
  • Fungal Infections (e.g., Candidiasis, Tinea Cruris): Although less common in this context, fungal infections can cause itching and swelling in the genital and anal areas.
• Do Not Miss Diagnoses
  • Syphilis: Despite the negative tests for other sexually transmitted infections (STIs), syphilis can present with a wide range of symptoms, including genital ulcers or rashes, and must be considered due to its potential severity and the fact that it can be missed if not specifically tested for.
  • Herpes Simplex Virus (HSV): Genital herpes can cause painful blisters or sores, but in some cases, it may present with milder symptoms such as itching or swelling, making it a crucial diagnosis not to miss.
  • Lymphogranuloma Venereum (LGV): A type of chlamydia infection that can cause symptoms like swollen lymph nodes in the groin, as well as genital and anal symptoms. Although the chlamydia test was negative, LGV may require specific testing to diagnose.
• Rare Diagnoses
  • Scabies: Caused by a mite, scabies can lead to intense itching and a pimple-like skin rash. It's less common in the genital area but should be considered, especially if the patient has been in close contact with someone infected.
  • Keratoderma Blennorrhagicum: A rare skin condition associated with reactive arthritis, which can cause skin lesions on the palms, soles, and occasionally the genitals.
  • Behçet's Disease: A form of vasculitis that can cause a wide range of symptoms, including genital ulcers and skin lesions. It's rare but important to consider due to its systemic implications.

Each of these diagnoses has a different set of implications for treatment and management, emphasizing the importance of a thorough diagnostic workup.