Snapping four days of gains, the Sensex at the Bombay Stock Exchange dropped 243 points, or 1.4 percent, to 17,676, today, after the RBI kept key interest rates unchanged. The 30-share Sensex had gained over 770 points in the past four trading sessions. The Nifty at the National Stock Exchange fell 83 points, or 1.5 percent, to 5,381.Stock markets in Japan, China, Hong Kong, South Korea and Singapore again ended mixed.