What defines a malignant tumor regarding its ability to invade other tissues?

A malignant tumor is primarily characterized by its ability to invade surrounding tissues and metastasize, which refers to the cancer's capacity to spread to distant organs or parts of the body through the bloodstream or lymphatic system. This invasive nature is a key distinction between malignant tumors and benign tumors.

In contrast to benign tumors, which are typically well-defined, localized, and grow slowly without invading nearby tissue, malignant tumors exhibit uncontrolled growth and can infiltrate adjacent tissues, disrupting normal cellular activity and function. The capability of metastasis is significant as it indicates the tumor's ability to establish secondary growths in different parts of the body, leading to more serious health implications.

Other options point to characteristics that are not relevant to defining malignancy. For instance, benign tumors (first choice) do not invade; they remain circumscribed and usually do not metastasize. Localized growth (third choice) is intrinsic to benign tumors, meaning it does not apply to malignant tumors. Normal cellular structure (fourth choice) refers to the appearance of cells, which in malignant tumors typically show abnormal cellular features, further supporting their aggressive nature. Thus, the defining feature of malignant tumors is their potential for metastasis, which underscores their invasive behavior.