**Question:** Spontaneously regressing tumours are

A. Indolent
B. Well-differentiated
C. Immune-mediated
D. Malignant

**Correct Answer:** C. Immune-mediated

**Core Concept:** Spontaneously regressing tumours are tumors that shrink and disappear without any treatment. This phenomenon can be explained by the involvement of the immune system in recognizing and eliminating the tumor cells.

**Why the Correct Answer is Right:** Spontaneous regression of tumors is a rare phenomenon primarily observed in benign or well-differentiated malignant tumors. In the case of immune-mediated regression, the immune system plays a crucial role in the elimination of tumor cells. One of the key mechanisms involves tumor-specific cytotoxic T cells (CD8+ T cells), which recognize and eliminate tumor cells through the release of cytotoxic granules containing perforin and granzymes.

**Why Each Wrong Option is Incorrect:**

A. Indolent tumors are generally slow-growing and non-aggressive, whereas spontaneous regression typically occurs in rapidly growing tumors. Thus, indolent tumors are not the correct answer.

B. Well-differentiated tumors are characterized by a high degree of cell differentiation and a low risk of progression to a more malignant state. Although well-differentiated tumors may occasionally regress, the correct answer is immune-mediated regression.

D. Malignant tumors are characterized by uncontrolled cell growth, invasion, and potential for distant spread. Malignant tumors do not typically regress spontaneously, making this option incorrect.

**Clinical Pearl:** Understanding immune-mediated tumor regression is crucial for understanding the tumor-immune system interaction. In some cases, the immune system can recognize and eliminate tumor cells, providing insight into potential therapeutic approaches, such as immune checkpoint inhibitors, to boost the immune response against cancer cells.