Chlamydia trachomatis is the Answer
In a young adult woman with pelvic inflammatory disease who tests negative for gonorrhea but positive for another common bacterial cause, the organism is Chlamydia trachomatis—the most frequently reported infectious disease in the United States and a primary causative agent of PID. 1
Why Chlamydia is the Correct Answer
C. trachomatis is explicitly identified as the most common bacterial sexually transmitted organism causing PID in the United States, with recovery rates from the cervix ranging from 5-39% of women diagnosed with PID and serologic evidence found in 20-40% of women with PID history. 1 The CDC guidelines consistently list C. trachomatis alongside N. gonorrhoeae as the two primary sexually transmitted organisms causing PID, with 10-40% of untreated cervical infections progressing to symptomatic upper genital tract disease. 2
The clinical presentation described—lower abdominal pain, fever, and abnormal vaginal discharge with a positive NAAT—is classic for chlamydial PID. 3 Chlamydia commonly causes endocervicitis that ascends via direct canalicular spread from the endocervix to the endometrium and fallopian tubes. 1, 4
Why the Other Options Are Incorrect
Gardnerella vaginalis is associated with bacterial vaginosis and can be isolated from the upper genital tract in 25-50% of acute PID cases, but it is part of polymicrobial infections rather than being a primary sexually transmitted causative organism. 1 It represents endogenous vaginal flora, not the "common cause" referenced in the question. 5
Mycoplasma genitalium has been recovered from the genital tract, but its role in PID remains less clear according to CDC guidelines. 1 While more recent research suggests M. genitalium may cause PID, standardized diagnostic tests are not commercially available, and it is not listed among the primary recognized etiologic agents in established guidelines. 1, 5
Clinical Implications for Management
Empiric treatment should cover C. trachomatis with either azithromycin 1g orally as a single dose OR doxycycline 100mg orally twice daily for 7 days. 1 For PID specifically, the recommended outpatient regimen includes a cephalosporin injection followed by doxycycline for 14 days, with metronidazole added if bacterial vaginosis or trichomoniasis is present. 3
Annual screening of all sexually active women under age 25 is recommended, as asymptomatic infection is common and untreated chlamydial cervicitis leads to serious sequelae including infertility (12% after one PID episode, 25% after two, and over 50% after three or more episodes) and ectopic pregnancy (sevenfold increased risk). 1, 2
Sex partners must be notified, examined, and treated, with abstinence from sexual intercourse until both partners complete therapy to prevent reinfection. 1