Which of the following best defines a tumor?

• A. A malignant cell
• B. A mass of abnormal cells that develops when cancerous cells divide and grow uncontrollably
• C. An immune cell
• D. A normal tissue

Answer: (B)

The idea being tested is what defines a tumor: a mass formed by abnormal, uncontrolled cell growth. Among the options, the description that a tumor is a mass of abnormal cells that develops when cells divide and grow uncontrollably best captures this concept. A single malignant cell isn’t a tumor by itself, and immune cells or normal tissue don’t form such an abnormal proliferating mass. It’s also important to note that tumors can be benign or malignant—the term describes the growth as a mass of proliferating cells, while behavior like invasion or metastasis distinguishes malignant from benign growth.