**Question:** The commonest cause of non-gonococcal urethritis is -

A. Chlamydia trachomatis
B. Neisseria gonorrhoeae
C. Mycoplasma genitalium
D. Trichomonas vaginalis

**Core Concept:**
Non-gonococcal urethritis refers to urethritis caused by pathogens other than Neisseria gonorrhoeae. The common causes include:

**Why the Correct Answer is Right:**
A. Chlamydia trachomatis is the most frequent cause of non-gonococcal urethritis. It is a Gram-negative bacterium that can infect the male reproductive system, leading to urethritis. Chlamydia trachomatis primarily affects the urethra, causing symptoms such as dysuria, urethral discharge, and burning sensation upon urination.

**Why Each Wrong Option is Incorrect:**
B. Neisseria gonorrhoeae (Disease: Gonorrhea) is the correct answer for gonococcal urethritis. Non-gonococcal urethritis is caused by other pathogens, including Chlamydia trachomatis.
C. Mycoplasma genitalium is a less common cause of non-gonococcal urethritis. While it is true that Mycoplasma genitalium can cause urethritis, Chlamydia trachomatis is the most frequent cause.
D. Trichomonas vaginalis is the pathogen responsible for trichomoniasis, a separate infection from non-gonococcal urethritis. Trichomonas vaginalis is an anaerobic flagellated protozoan parasite causing symptoms such as urethritis, dysuria, and urethral discharge.

**Clinical Pearl:**
Chlamydia trachomatis is a significant pathogen in sexually transmitted infections (STIs) and should always be considered when assessing patients with urethritis symptoms, especially those with a history of unprotected sex or contact with an infected individual. Timely diagnosis and treatment can prevent complications like pelvic inflammatory disease, ectopic pregnancy, and reactive arthritis.

**Correct Answer:** C. Mycoplasma genitalium is a less common cause of non-gonococcal urethritis, but the most frequent cause is Chlamydia trachomatis.