31. After successive surgery and chemotherapy, the tumor of a breast cancer patient subsided. However after almost 5 years, the tumor relapsed in a more aggressive manner and did not respond to the conventional chemotherapy delivered earlier.

The following postulations were made.
A. Chemo-resistant cells were persisting within the tumor even after therapy.
B. A population of quiescent cells existed, which under favorable conditions, transformed to new tumor cells.
C. High ABC (ATP-Binding Cassette) transporter expressing cells persisted in the breast during
chemotherapy.
D. Breast tumor cells which may have migrated to other tissues, returned to the breast
immediately after chemotherapy was terminated.

Which of the above combination of statements is true?
(1) A and D                                             (2) A, B and C
(3) only B                                                (4) B and D

The correct combination of statements explaining the breast cancer tumor relapse after initial treatment is:

(2) A, B and C

Explanation:

• A. Chemo-resistant cells persisting within the tumor:
  Even after surgery and chemotherapy, some cancer cells can survive because they resist treatment, leading to relapse.​

• B. A population of quiescent cells existing, which can later transform into new tumor cells:
  Dormant or quiescent cancer cells can evade therapy and reactivate under favorable conditions, causing recurrence.​

• C. High ABC transporter-expressing cells persisting during chemotherapy:
  Cells expressing high levels of ATP-Binding Cassette (ABC) transporters actively efflux drugs, contributing to chemotherapy resistance and tumor recurrence.​

• D. Breast tumor cells migrating to other tissues and returning to the breast is not a recognized mechanism of relapse; metastasis leads to colonization of distant sites rather than migration back to the original site [general cancer biology knowledge].