参考答案：C

解析：Factors related to growth and maturation significantly alter an individual's capacity to metabolize and excrete drugs. Thus, the premature infant is at risk for problems because of immaturity. Deficiencies associated with immaturity become more important with decreasing age. Enzyme systems develop quickly, with most increasing to adult levels within I to 8 weeks after birth. Within the 1st year of life, all are probably as active as they will ever be.