The modern era of clinical organ transplantation began with the advent of chemical immunosuppression. The important drug discovery that produced the initial success of cadaveric transplantation was:

B. Azathioprine.

DISCUSSION: All of the listed drugs have immunosuppressive activity that has proved useful in transplant recipients. However, the discovery in 1959 by Schwartz and Dameshek that 6-mercaptopurine blocked antibody production and the subsequent creation by Hitchings in 1961 of its safe, convenient imidazole derivative named azathioprine produced the first consistently effective immunosuppression for successful cadaveric renal transplantation.