Okay, let's tackle this question. The question is about the commonest cause of an obliterative stricture of the membranous urethra. I need to recall the different causes of urethral strictures and which part of the urethra they affect.

First, I remember that urethral strictures can occur due to trauma, infection, or congenital issues. The urethra is divided into different parts: the prostatic, membranous, and spongy (penile) portions. The membranous urethra is the shortest part, located between the prostate and the urogenital diaphragm.

Common causes of strictures in the membranous urethra... Hmm. I think that trauma, especially from urethral instrumentation like catheterization, is a big one. Also, pelvic fractures can cause injury here. Another possibility is infection, like gonococcal urethritis leading to scarring. But wait, which is the most common?

Wait, I remember that straddle injuries, which affect the bulbous part of the urethra, are more common in the spongy portion. But for the membranous part, pelvic fractures are a major cause. Pelvic fractures can cause avulsion of the urethra at the membranous segment, leading to a stricture. So maybe pelvic trauma is the commonest cause here.

Let me check the options. The correct answer given is Option C: Pelvic fracture. So the other options would be things like urethritis, catheterization, or congenital causes. But why is pelvic fracture the most common? Because the membranous urethra is in a fixed position near the pelvic bones, so when there's a fracture, the movement of the pelvic bones can tear the urethra. This leads to scarring and stricture formation.

Other options like catheter injury (Option B) might cause strictures in the spongy urethra, not the membranous. Infections like gonorrhea (Option A) usually affect the anterior urethra. Congenital strictures (Option D) are rare. So the key is understanding the location of the injury. Pelvic trauma directly affects the membranous segment, making it the most common cause.

**Core Concept**
Obliterative stricture of the membranous urethra typically results from **trauma** or **inflammation**. The membranous urethra, located between the prostate and urogenital diaphragm, is particularly vulnerable to injury from **pelvic fractures**, which can cause avulsion or direct damage due to bony displacement.

**Why the Correct Answer is Right**
**Pelvic fracture** is the most common cause of membranous urethral stricture. During high-energy pelvic trauma, the pubic bones may displace anteriorly, tearing the fixed membranous urethra. This leads to **urethral avulsion**, followed by fibrosis and stricture formation. The retroperitoneal location of the membranous urethra makes it less accessible to spontaneous healing compared to the spongy urethra.

**Why Each Wrong Option is Incorrect**
**Option A: Gonococcal urethritis** – Typically causes anterior urethral strictures via scarring, not the membranous segment.

✓ Correct Answer: D. Road-traffic accident with fracture pelvis and rupture urethra