Sentence:  The senior policy officer may be moved to think hard about a problem by any of an infinite variety of stimuli : an idea in his own head , the suggestions of a colleague , a question from the Secretary or the President , a proposal by another department , a communication from a foreign government or an American ambassador abroad , the filing of an item for the agenda of the United Nations or of any other of dozens of international bodies , a news item read at the breakfast table , a question to the President or the Secretary at a news conference , a speech by a Senator or Congressman , an article in a <WSD>periodical</WSD> , a resolution from a national organization , a request for assistance from some private American interests abroad , et cetera , ad infinitum . 
Meaning of the word marked with <WSD> tags:  a publication that appears at fixed intervals