A 75 years old male needs catheterization of bladder due to Benign Hypertrophy of Prostate (BPH). Which is the least dilatable part of urethra? (A) Prostatic part (C) Penile part (B) Membranous part (D) Prostatic and Penile part
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The answer is confirmed by multiple authoritative sources.
Answer: (B) Membranous Part
The intermediate (membranous) urethra is the narrowest and least distensible (least dilatable) part of the male urethra.
Why the Membranous Urethra?
The membranous urethra passes through the deep perineal pouch, is tightly surrounded by the circular fibers of the external urethral sphincter, and penetrates the perineal membrane. These structural features make it:
- Very short (only 1.0-1.5 cm)
- Enclosed by tough fascial layers and skeletal muscle
- Rigid and resistant to dilation
Comparison of All Parts (Male Urethra)
| Part | Length | Key Feature |
|---|---|---|
| Intramural (preprostatic) | 0.5-1.5 cm | Surrounded by internal urethral sphincter |
| Prostatic | 3.0-4.0 cm | Widest and most dilatable part |
| Membranous | 1.0-1.5 cm | Narrowest and least distensible |
| Spongy (penile) | ~15 cm | Longest and most mobile part |
Clinical Relevance in BPH
In this 75-year-old male with BPH, the prostatic urethra is obstructed by the enlarged prostate. During urethral catheterization, the membranous urethra presents the most resistance to passage. Forcing a catheter through it can cause urethral injury or false passage. A coude (curved-tip) catheter is typically used in men with BPH to navigate the altered prostatic angle while being gentle at the membranous segment.
Note: The external urethral orifice (meatus) is technically the absolute narrowest point, which is why the textbook specifies "narrowest and least distensible except for the external urethral orifice." Among the answer options provided, the membranous part is the correct answer.
Sources: Fischer's Mastery of Surgery 8th ed., Table 281.1; Campbell-Walsh-Wein Urology; Grainger & Allison's Diagnostic Radiology
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