32. While testing the effect of several potent anti-cancer compounds on cycling human oral cancer cells, a student observed that a major percentage of cells showed dose-deep cell death after 12 hours of drug treatment. However the remaining cells repopulated the culture dish once the compounds were removed and the cells were cultured in complete medium.

The student made the following assumptions:
A. Not all cells were equally affected by the compounds as they were not synchronized before
treatment.
B. The compound selectively killed cells which were in G_o phase.
C. The cancer stem cells were impervious to the effects of the compounds and therefore repopulated the culture.
D. The cancer cells differentiated into a mesenchymal phenotype and grew in fresh culture medium containing inhibitors of epithelial-to-mesenchymal transition (EMT).

Which one of the following combination of assumptions would best justify the results?
(1) B and C                                              (2) A and C
(3) B and D                                              (4) A and B

he best justification for the observed results in the experiment is:

(2) A and C

Explanation:

• A. Not all cells were equally affected as they were not synchronized:
  Cancer cells in different phases of the cell cycle respond differently to drugs; lack of synchronization means some cells survive treatment, explaining partial killing.​

• C. Cancer stem cells were impervious to the compounds and repopulated the culture:
  CSCs often show resistance to chemotherapy due to mechanisms including drug efflux pumps, efficient DNA repair, and dormancy. These CSCs survive and can regenerate the tumor population once the drug is removed.​

• B. The compound selectively killing cells in Go phase:
  This is unlikely, as chemotherapy mainly targets proliferating cells rather than quiescent (Go) cells.

• D. EMT-related differentiation and growth in the presence of EMT inhibitors:
  EMT promotes drug resistance and metastasis, but growth under EMT inhibitors contradicts EMT contribution to repopulation here.

Thus, the combination of heterogeneous drug susceptibility (A) and chemotherapy-resistant CSC survival (C) best explains the dose-dependent cell death followed by culture repopulation.