Dow Knocked Lower by Merck's Drug Recall  NEW YORK (Reuters) - Drug maker Merck   Co. Inc. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=MRK.N target=/stocks/quickinfo/fullquote"&gt;MRK.N&lt;/A&gt;  dominated trading in U.S. stocks on Thursday, single-handedly  driving the 30-stock Dow Jones average lower after it withdrew  its arthritis drug Vioxx because a study showed it doubled the  risk of heart attack and stroke.