**Core Concept**
The ureters are retroperitoneal structures that traverse the abdominal cavity, crossing several key anatomical landmarks. Understanding their course is crucial for interpreting abdominal radiographs and diagnosing potential obstructions.

**Why the Correct Answer is Right**
The ureter crosses the psoas major muscle, a major muscle of the posterior abdominal wall. This is a result of the ureter's retroperitoneal course, where it runs between the peritoneum and the posterior abdominal muscles. The psoas major muscle is a key landmark for identifying the ureter's position on an abdominal radiograph. As the ureter ascends towards the kidney, it crosses the psoas major muscle, which can be visualized on a radiograph.

**Why Each Wrong Option is Incorrect**
**Option A:** The inferior vena cava is a major venous structure that crosses the midline in the abdominal cavity but is not directly associated with the ureter's course.
**Option B:** The aorta is a major arterial structure that descends through the abdominal cavity but is not directly crossed by the ureter.
**Option C:** The iliac crest is a bony landmark that forms the roof of the pelvic cavity but is not directly associated with the ureter's course.

**Clinical Pearl / High-Yield Fact**
On an abdominal radiograph, the ureter can be visualized crossing the psoas major muscle, which is a key landmark for identifying potential obstructions or other ureteral pathologies.

**Correct Answer:** C. The psoas major muscle.