one of后如果是特指某一范围的事物,要用THE.如果是泛指,不用THE.如: 1) There are 10 boys in the classroom, and one of the boys is my boyfriend. 2) For me, one of happinesses is having a big wallet hit my head. 在你的问题中, 用one of simple sugars和one of the simple sugars都是可以的,但是意思略有不同.用前者,后面的used in food preparations for infants修饰one, 原句其实就是Lactose, a sugar presenting in milk, is a simple sugar which is used in food preparations for infants.(其它simpe sugar能不能用于food preparations for infants我们不知道). 如果用后者,后面的部分修饰simple sugars,因为simple sugars被修饰后,是在某一限定范围,所以前面要加THE.这时句子的意思成了Loctose是用于food preparations for infants的simple sugar的一种. | 
