A 28-year-old woman presents with lower abdominal pain, purulent vaginal discharge, and cervical motion tenderness. Pelvic inflammatory disease is suspected. Which is the most common causative organism in PID?

Explanation

## Microbiology of Pelvic Inflammatory Disease **Key Point:** Chlamydia trachomatis is the most common causative organism of PID, accounting for approximately 40–50% of cases, followed closely by Neisseria gonorrhoeae. ### Epidemiology and Frequency | Organism | Frequency | Key Features | |----------|-----------|---------------| | Chlamydia trachomatis | 40–50% (most common) | Often asymptomatic cervicitis; ascending infection | | Neisseria gonorrhoeae | 30–40% | Acute presentation; more symptomatic | | Mycoplasma genitalium | 5–10% | Emerging pathogen; resistant strains | | Ureaplasma urealyticum | <5% | Rare as sole pathogen | | Polymicrobial (anaerobes) | Common | Especially in advanced/chronic PID | **High-Yield:** Chlamydia is the leading cause because: 1. It causes **silent/asymptomatic cervicitis** — women often do not seek early treatment 2. Ascending infection occurs silently, delaying diagnosis 3. Higher prevalence in sexually active young women (15–25 years) **Clinical Pearl:** Gonorrhea causes more **acute, symptomatic** PID (purulent discharge, acute pain), making it more likely to be diagnosed early. Chlamydia's insidious nature leads to delayed presentation and higher rates of chronic sequelae (infertility, ectopic pregnancy, chronic pelvic pain). ### Why Chlamydia Dominates - **Intracellular pathogen** → evades immune response → persistent infection - **Serovars D–L** specifically infect the urogenital tract - Often **co-infected with gonorrhea** (10–40% of cases), but Chlamydia is the sole pathogen in ~50% of PID cases - Screening and treatment gaps in resource-limited settings increase prevalence [cite:Berek & Novak's Gynecology 16e Ch 12]