A tumor marker is best defined as:

A tumor marker is best defined as a molecule that indicates tumor origin or activity. These markers are often substances produced by cancer cells or by the body in response to cancer, and they can be found in blood, urine, or tissue samples. The presence or elevated levels of these markers can help in diagnosing cancer, monitoring treatment effectiveness, and assessing the likelihood of recurrence.

Understanding the significance of tumor markers is crucial because they can provide valuable information about the type and progression of cancer, which can aid in developing a tailored treatment plan for patients. Tumor markers may include proteins, hormones, or gene mutations, and their analysis can enhance the specificity of cancer diagnosis.

The other options lack the core definition of tumor markers. A malfunctioning tumor suppressor gene refers to genetic aspects of cancer, while a byproduct of healthy cell division does not relate specifically to cancerous processes. A measure of immune response to cancer, on the other hand, reflects how the body is responding to cancer rather than indicating the cancer itself. Therefore, the chosen definition encapsulates the primary function and importance of tumor markers in oncology.