A 28-year-old sexually active male presents with burning micturition. On clinical examination, there is no ulcer in the genitals. Urine examination shows 50 WBCs/HPF, no RBCs, leucocyte esterase positive, gonococcal culture negative. What could be the most probable causative organism

**Core Concept**
The patient's symptoms, particularly burning micturition and the presence of white blood cells (WBCs) in the urine, suggest a urinary tract infection (UTI). The absence of red blood cells (RBCs) and gonococcal culture negativity point towards a non-gonococcal cause. Leucocyte esterase positivity further supports the presence of an infection.

**Why the Correct Answer is Right**
The most probable causative organism in this case is **Escherichia coli (E. coli)**, which is a common gram-negative bacteria responsible for approximately 80-85% of uncomplicated UTIs in sexually active young adults. The presence of WBCs and leucocyte esterase positivity in the urine indicates an inflammatory response, likely caused by bacterial invasion. E. coli adheres to the uroepithelial cells through fimbriae, leading to the formation of biofilms and subsequent inflammation.

**Why Each Wrong Option is Incorrect**

* **Option A:** Proteus mirabilis is a gram-negative bacteria that can cause UTIs, particularly in individuals with urinary tract abnormalities or catheterization. However, it is not the most common cause of UTIs in sexually active young adults.
* **Option B:** Klebsiella pneumoniae is another gram-negative bacteria that can cause UTIs, but it is less common than E. coli in this demographic.
* **Option C:** Pseudomonas aeruginosa is a gram-negative bacteria that can cause UTIs, particularly in immunocompromised individuals or those with urinary tract abnormalities. However, it is not the most probable causative organism in this case.

**Clinical Pearl / High-Yield Fact**
In sexually active young adults with symptoms of UTI, it is essential to rule out gonococcal infection, but also consider non-gonococcal causes, such as E. coli, which is the most common cause.

**Correct Answer:** D.

✓ Correct Answer: C. Chlamydia trachomatis