Final answer:
The trigone area at the base of the posterior bladder wall is free of rugae and stays smooth, ensuring free flow of urine out of the bladder.
Step-by-step explanation:
The area on the posterior bladder wall that is free of rugae is known as the trigone area. This region is located at the base of the bladder and is marked by the two ureters entering on the posterior-lateral aspect and the urethral orifice at the anterior aspect, forming the points of a triangle. Unlike the rest of the bladder lining, which includes transitional epithelium that can stretch and create rugae (folds) when the bladder is empty, the trigone area remains smooth even when the bladder is full. The smoothness of the trigone is important because it helps ensure that urine can flow freely out of the bladder without being trapped in folds.