This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Transplantation, Subject name: Surgery
As compared with the early immunosuppressive drugs (azathioprine, steroids, anti-lymphocyte serum) some newer agents have the following specific advantages: 
Select and explain. 
A. Cyclosporine, which interferes with lymphokine production, exhibits neither bone marrow nor renal toxicity, B. Monoclonal antibody (OKT3) is more available and has greater specificity and fewer side effects than anti-lymphocyte serum, C. Tacrolimus (FK506) has propeies similar to those of cyclosporine but is especially valuable for rescue of grafts that are failing on cyclosporine therapy, D. None of the above
C: Tacrolimus (FK506) has propeies similar to those of cyclosporine but is especially valuable for rescue of grafts that are failing on cyclosporine therapy
Answer: C: Tacrolimus (FK506) has propeies similar to those of cyclosporine but is especially valuable for rescue of grafts that are failing on cyclosporine therapy 
Explanation: Cyclosporine interferes with the production of cytokines and lacks the bone marrow toxicity of azathioprine. Unfounately, its chief toxicity is renal. Although OKT3 is more available, more uniform, and more specific than antilymphocyte serum, some of its side effects are even greater, such as fever, chills, nausea, vomiting, diarrhea, and pulmonary edema. Tacrolimus has been used most extensively for liver grafts. It has been found especially valuable in reversing rejection of failing grafts.