5.Which of the following is most likely to enhance a cell’s toxicity to methotrexate?

A. Reduced expression of reduced folate carrier (RFC) protein on the cell membrane

B. Increased expression of ATP binding cassette efflux (ABC) protein on the cell membrane

C. Increased expression of dihydrofolate reductase (DHFR)

D. Increased expression of folylpolyglutamyl synthetase (FPGS)