**Core Concept**
CA 125 is a tumor marker that is often elevated in patients with epithelial ovarian cancer, particularly those with serous or mucinous histologies. It is a mucin-type glycoprotein that is expressed on the surface of ovarian cancer cells. Elevated levels of CA 125 can be detected in the blood and are used as a marker for ovarian cancer progression or recurrence.

**Why the Correct Answer is Right**
The primary use of CA 125 is for monitoring the response to treatment and detecting recurrence in patients with ovarian cancer. Serial measurements of CA 125 levels can help clinicians assess the effectiveness of therapy and identify patients who may be experiencing tumor progression or recurrence. This is particularly useful in patients with advanced ovarian cancer, where tumor markers can provide valuable information about disease status. The levels of CA 125 can be elevated in other conditions, such as endometriosis, pelvic inflammatory disease, or benign ovarian tumors, but in the context of ovarian cancer, it is a useful marker for disease monitoring.

**Why Each Wrong Option is Incorrect**
**Option B:** While CA 125 can be elevated in some patients with pancreatic cancer, it is not a specific or sensitive marker for this disease. Other tumor markers, such as CA 19-9, are more commonly used for pancreatic cancer diagnosis and monitoring.
**Option C:** CA 125 is not typically elevated in stomach cancer, and other tumor markers, such as CEA, are more commonly used for this disease.
**Option D:** While CA 125 is a marker for ovarian cancer, it is not used for diagnosis, but rather for monitoring disease progression or recurrence.

**Clinical Pearl / High-Yield Fact**
CA 125 levels can fluctuate in response to menstruation, pregnancy, or other physiological changes, which can lead to false-positive results. Clinicians should interpret CA 125 levels in the context of the patient's clinical presentation and medical history.

**✓ Correct Answer: A. Follow up of ovarian cancer**