What is Urethral prolapse in a dog?

Urethral prolapse is a condition where the mucosal lining of the urethra falls out of place outside of the body, making it visible. This condition can very often affect other organs, such as the urinary bladder, the urinary tract, the reproductive organs, and of course the immune system.
The prolapse urethra, very often looks like a pea-sized mass, and can have a bright red or purple color.