A 28-year-old woman presents with mucopurulent cervical discharge and dysuria. Gram stain of the cervical swab shows no intracellular gram-negative diplococci. Nucleic acid amplification test (NAAT) is positive. Which is the most common cause of non-gonococcal urethritis (NGU) in this patient?

Explanation

## Non-Gonococcal Urethritis (NGU) — Etiology **Key Point:** Chlamydia trachomatis is responsible for 30–50% of all NGU cases and remains the single most common bacterial cause of urethritis when gonorrhea is excluded [cite:Park 26e Ch 30]. ### Clinical Context The patient has: - Mucopurulent urethral/cervical discharge - Dysuria - Negative Gram stain (no intracellular gram-negative diplococci, ruling out Neisseria gonorrhoeae) - Positive NAAT (nucleic acid amplification test — highly sensitive and specific for chlamydia) ### Epidemiology of NGU Causes | Organism | Frequency | Key Features | |----------|-----------|---------------| | **Chlamydia trachomatis** | 30–50% | Most common; serovars D–K; intracellular; NAAT gold standard | | Ureaplasma urealyticum | 10–15% | Fastidious; requires special culture media; less common | | Mycoplasma genitalium | 5–10% | Emerging pathogen; difficult to culture; rare in most settings | | Trichomonas vaginalis | 5–10% | Protozoan; motile trophozoites on wet mount; causes frothy discharge | **High-Yield:** In India and globally, C. trachomatis (serovars D–K) accounts for the majority of NGU cases. Serovars L1, L2, L3 cause lymphogranuloma venereum (LGV), a more invasive syndrome. ### Why NAAT Positivity Points to Chlamydia - NAAT (PCR, TMA, SDA) is the gold standard for chlamydial detection - Sensitivity and specificity both >95% - Detects both viable and non-viable organisms - Ureaplasma and Mycoplasma require specialized PCR assays not routinely used in standard NGU workup **Clinical Pearl:** In a woman with NGU and positive NAAT, empiric treatment with azithromycin or doxycycline covers C. trachomatis and is first-line. Partner notification and treatment are mandatory. ### Differential Reasoning - **Ureaplasma urealyticum:** Second most common cause of NGU but requires special culture and is less reliably detected by routine NAAT - **Mycoplasma genitalium:** Emerging cause; not routinely screened; requires specialized PCR - **Trichomonas vaginalis:** Would show motile trophozoites on wet mount; causes frothy, malodorous discharge; detected by different methods (wet mount, culture, NAAT)