A 28-year-old man presents with urethral discharge and dysuria for 5 days. Gram stain of urethral smear shows gram-negative diplococci within polymorphonuclear leukocytes. What is the most common cause of non-gonococcal urethritis (NGU) in this patient if gonorrhea is ruled out?

Explanation

## Etiology of Non-Gonococcal Urethritis (NGU) **Key Point:** Chlamydia trachomatis is responsible for 30–50% of all NGU cases and is the most common bacterial cause of urethritis when gonorrhea is excluded. ### Epidemiology and Frequency | Organism | Frequency in NGU | Gram Stain | Culture Method | |----------|-----------------|-----------|----------------| | Chlamydia trachomatis | 30–50% | Intracellular gram-negative | Cell culture / NAAT | | Ureaplasma urealyticum | 10–15% | Not visible on Gram stain | Special media | | Mycoplasma genitalium | 5–10% | Not visible on Gram stain | PCR / NAAT | | Trichomonas vaginalis | 5–10% | Motile trophozoite | Wet mount / culture | **High-Yield:** The definition of NGU itself is urethritis with negative gonorrhea culture or Gram stain — Chlamydia is the leading cause in this category. ### Clinical Features of Chlamydial Urethritis 1. **Incubation period:** 7–14 days (longer than gonorrhea's 2–5 days) 2. **Discharge:** Mucopurulent, often scant and clear/whitish 3. **Dysuria:** Present but often milder than in gonorrhea 4. **Gram stain:** Intracellular gram-negative diplococci (but may be sparse; negative stain does NOT rule out chlamydia) 5. **Diagnosis:** NAAT (nucleic acid amplification test) is gold standard; culture is insensitive and requires special media **Clinical Pearl:** Chlamydia often causes **persistent or recurrent urethritis** after treatment of gonorrhea — the two infections frequently coexist, and failure to treat chlamydia leads to post-gonococcal urethritis (PGU). ### Why Chlamydia Dominates NGU - **Prevalence:** Most common sexually transmitted bacterial pathogen globally - **Asymptomatic carriage:** Up to 50% of infected individuals are asymptomatic, allowing silent transmission - **Fastidious organism:** Requires intracellular culture; easily missed on routine Gram stain - **Ascending infection risk:** Can cause pelvic inflammatory disease (PID), epididymitis, and proctitis if untreated **Mnemonic:** **C-NGU** = **C**hlamydia is the **N**umber one cause of **N**on-**G**onococcal **U**rethritis. ### Treatment Implications - **First-line:** Azithromycin 1 g single dose OR doxycycline 100 mg BD for 7 days - **Partner notification:** Essential; sexual partners must be treated empirically - **Test of cure:** Not routinely recommended; repeat NAAT after 3 weeks if symptoms persist