(C) Inferior mesenteric artery
A horseshoe kidney forms when the inferior poles of two kidneys fuse during development. As the kidneys rise from the pelvis, they encounter the inferior mesenteric artery and cannot rise to the normal level in the abdomen. These patients are typically asymptomatic if they have no other abnormalities.
Answer A is incorrect. The aorta would not obstruct the path of a rising horseshoe kidney during development.
Answer B is incorrect. The celiac trunk leaves the aorta at a level above the location of normally developed kidneys, and thus cannot be responsible for the low location of a horseshoe kidney.
Answer D is incorrect. The inferior vena cava would not obstruct the path of a rising horse-shoe kidney during development.
Answer E is incorrect. The superior mesenteric artery leaves the aorta at the level where normally developing kidneys are located, and thus it cannot be responsible for the low level of a horseshoe kidney.