Plicae Colliculi
C E D B A The Plicae colliculi are the thin fins of mucosa that extend from the bottom of the verumontanum (or Colliculus seminalis).  These thin fins of tissue extend from the prostatic urethra and terminate in the membranous urethra. They are variable in appearance and often not visible during voiding urethrography. One can think of them as the least apparent end of the spectrum that extends to the entity known as posterior urethral valves.  When they enlarge they may be quite prominent but rarely cause a true obstruction.   Notice that the posterior urethras in the following images are not dilated, and the anterior or distal urethra is normal in caliber as well. 

See the below topics for additional information

Anson & McVay Surgical Anatomy 6th ed.  Chester B McVay  W.B. Saunders Co.
A. Sphincter
B. Site of the verumontanum
C. Plicae colliculi
D. Membranous urethra
E. Penile urethra
 
Differential Diagnosis
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Cobb’s collar
Urethral diaphragm
Posterior urethral valves