Why is masking needed?

What vessel is the arrow pointing to in the image below?

Identify the uterus.

Click on the letter that corresponds to the pubic symphesis.

Click on the letter that corresponds to the sacro-iliac joint.

Identify the rectum.

Click on the letter that corresponds to the acetabulum.

Click on the chorocoid process.

Click on the left ventricle.

Click on the superior vena cava.