**Core Concept**
The urinary bladder is a complex organ with distinct anatomical and histological features. The trigone, a triangular area at the base of the bladder, has unique characteristics that differentiate it from the rest of the bladder. Understanding the histological composition of the trigone is crucial for accurate clinical diagnosis and surgical planning.

**Why the Correct Answer is Right**
The trigone is an area of the bladder that is formed by the two ureteric orifices and the internal urethral orifice. It does not have a muscularis mucosa, which is a layer of smooth muscle that is present in other parts of the bladder. This absence of muscularis mucosa makes the trigone a smooth, non-contractile area. The absence of muscularis mucosa in the trigone is a key distinguishing feature from the rest of the bladder.

**Why Each Wrong Option is Incorrect**
**Option A:** The interureteric ridge, also known as the bar of Mercier, is a ridge of tissue that separates the two ureteric orifices in the trigone. This statement is true, so it is not the correct answer.

**Option C:** The trigonal muscle is actually absent in the trigone, not present. The trigone is a smooth area that does not have any significant muscular layers.

**Option D:** The uvula vesicae is a mucosal fold that is formed by the base of the median lobe of the prostate, but it is not related to the trigone of the bladder. This statement is not relevant to the anatomy of the trigone.

**Clinical Pearl / High-Yield Fact**
When examining the bladder, it is essential to recognize that the trigone is a smooth, non-contractile area that lacks muscularis mucosa. This knowledge is critical for accurate diagnosis and surgical planning, as the trigone is a common site for bladder tumors and injuries.

**✓ Correct Answer: B. Muscularis mucosa is absent in Trigone making it smooth**