What characterizes a malignant tumour?

A malignant tumour is characterized by its ability to invade and destroy nearby normal tissues. This aggressive behavior is due to the cancerous cells' uncontrolled growth and division, which allows them to penetrate surrounding tissues. Malignant tumours can also shed cells into the bloodstream or lymphatic system, leading to metastasis, where cancer spreads to other parts of the body.

In contrast, the other descriptions present characteristics of benign tumours or non-cancerous growths. For instance, tumours that grow slowly and do not spread are typically benign. Benign tumours are localized and do not invade surrounding tissues, nor do they pose the same level of threat to health as malignant tumours. Additionally, a benign tumour is defined by its non-harmful nature, while a malignant tumour is, by definition, harmful and can be life-threatening. The assertion that a tumour remains limited to its original tissue applies to benign tumours as well, highlighting that they do not invade or metastasize.

Thus, the defining characteristic of a malignant tumour is its invasive and destructive growth pattern.