**Core Concept**
The ureter's entry into the bladder is a critical anatomical feature that plays a significant role in the functioning of the urinary system. The ureter's insertion into the bladder is characterized by a specific angle, which has important implications for the prevention of vesicoureteral reflux.

**Why the Correct Answer is Right**
The ureter enters the bladder at the lateral angle of the trigone, which is a triangular area formed by the two ureteric orifices and the internal urethral orifice. This entry is facilitated by the formation of a ureterovesical junction, where the ureter inserts into the bladder wall. The ureter's entry at the lateral angle of the trigone is crucial in preventing vesicoureteral reflux, a condition where urine flows back up the ureter from the bladder. This is due to the fact that the ureter's entry is at a 90-degree angle to the bladder's internal surface, which creates a natural anti-reflux mechanism.

**Why Each Wrong Option is Incorrect**
* **Option A:** The ureter does not enter the bladder at the medial angle of the trigone. The medial angle of the trigone is actually formed by the two ureteric orifices.
* **Option B:** While the ureter does enter the bladder at an angle, it is not the lateral angle of the trigone. This option is incorrect because it misidentifies the specific angle of entry.
* **Option C:** The ureter's entry into the bladder is not a straight line. The ureter's entry is facilitated by the formation of a ureterovesical junction, which involves a slight angle of entry.
* **Option D:** The ureter's entry into the bladder is not valveless. The ureterovesical junction is actually a site of high pressure and flow, which is regulated by the presence of a natural anti-reflux mechanism.

**Clinical Pearl / High-Yield Fact**
It is essential to recognize the importance of the ureter's entry into the bladder in preventing vesicoureteral reflux. This is particularly relevant in clinical scenarios where patients have undergone surgical interventions or have underlying conditions that may compromise the integrity of the ureterovesical junction.

**Correct Answer: 2. At lateral angle of trigone**