**Core Concept**
The ureter is a muscular tube that transports urine from the kidney to the bladder, and its course is closely associated with several pelvic structures. The ureter passes through the pelvis, where it lies against various muscles and other anatomical landmarks.

**Why the Correct Answer is Right**
The correct answer is the psoas major muscle, which is a long, flat muscle that extends from the lower back to the femur. As the ureter descends from the kidney to the bladder, it lies against the anterior surface of the psoas major muscle. This relationship is clinically important because it can be used to locate the ureter during surgical procedures or imaging studies. The psoas major muscle is a key landmark for identifying the ureter, especially in the retroperitoneal space.

**Why Each Wrong Option is Incorrect**
**Option A:** The iliacus muscle is a separate muscle that lies lateral to the psoas major muscle and is not directly associated with the ureter.
**Option B:** The obturator internus muscle is a small muscle that lies in the pelvic floor and is not in close proximity to the ureter.
**Option C:** The piriformis muscle is a muscle that lies in the lower back and is not directly associated with the ureter.

**Clinical Pearl / High-Yield Fact**
The psoas major muscle is a key landmark for identifying the ureter during surgical procedures or imaging studies. Remembering the relationship between the psoas major muscle and the ureter can help you locate the ureter more accurately.

**Correct Answer:** C.