**Core Concept**
The CA-125 antigen is a mucin-like glycoprotein that is overexpressed on the surface of various epithelial cancers, particularly ovarian cancer. It is used as a tumor marker for monitoring disease progression and response to treatment.

**Why the Correct Answer is Right**
The normal upper limit of CA-125 is typically considered to be 35 units/mL. Elevated levels of CA-125 can be seen in a variety of conditions, including ovarian cancer, endometriosis, and pelvic inflammatory disease. The enzyme linked immunosorbent assay (ELISA) is commonly used to measure CA-125 levels in the blood. The normal upper limit of CA-125 is used as a reference value to determine if a patient's levels are elevated and may indicate the presence of cancer or another underlying condition.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the normal upper limit of CA-125 is not 20 units/mL, which is too low and may not accurately reflect the range of normal values.
**Option B:** This option is incorrect because the normal upper limit of CA-125 is not 50 units/mL, which is too high and may not accurately reflect the range of normal values.
**Option C:** This option is incorrect because the normal upper limit of CA-125 is not 45 units/mL, which is too high and may not accurately reflect the range of normal values.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that elevated CA-125 levels can be seen in a variety of conditions, not just ovarian cancer. A normal CA-125 level does not rule out cancer, and a elevated level requires further investigation to determine the underlying cause.

**Correct Answer: C. 35 units/mL**