为啥arm前用an不用a

因为arm的首字母a是元音，例如：

1、He had had an arm amputated and relied on her to take down his books from dictation.他的一只胳膊截掉了，只能口授书的内容让她来记录。

2、He has lost the use of an arm.他的一只胳膊残废了。