Noun and の adjective
いし
ほんの短い時間だけでしたが、あの医師は二度テレビに出たことがあります。
That doctor appeared on TV twice, though just for short amounts of time.
A medicine expert is someone who knows a lot about medicine. Those people are (I hope) generally doctors.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.