**Core Concept**
CA 125 is a tumor marker primarily associated with ovarian cancer, but it can also be elevated in other conditions due to its expression in various tissues and its release into the bloodstream. The CA 125 antigen is a mucin-like glycoprotein that is overexpressed in many types of cancer, including epithelial ovarian cancer.

**Why the Correct Answer is Right**
CA 125 is commonly used as a surrogate marker for monitoring ovarian cancer treatment response and detecting recurrence. However, it can be elevated in conditions other than ovarian cancer, such as endometriosis, pelvic inflammatory disease, and benign ovarian cysts. This is because CA 125 is not specific to ovarian cancer and can be produced by other tissues, leading to false-positive results.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because CA 125 can be elevated in endometriosis, a condition characterized by the growth of endometrial tissue outside the uterus. In endometriosis, CA 125 levels can be elevated due to the expression of the antigen in endometrial tissue.
**Option B:** This option is incorrect because CA 125 can be elevated in benign ovarian cysts, such as functional ovarian cysts or cystadenomas. In these conditions, CA 125 levels can be elevated due to the release of the antigen from the cysts into the bloodstream.
**Option C:** This option is incorrect because CA 125 can be elevated in pelvic inflammatory disease (PID), a condition characterized by infection and inflammation of the female reproductive organs. In PID, CA 125 levels can be elevated due to the release of the antigen from inflamed tissues into the bloodstream.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that CA 125 is not a specific marker for ovarian cancer and can be elevated in various other conditions. Therefore, a single elevated CA 125 value should not be used in isolation to diagnose ovarian cancer, but rather as part of a comprehensive clinical evaluation.

**Correct Answer: D.**