The Sensex at the Bombay Stock Exchange dropped 161 points, or 0.9 percent, to close at 17,381, amid weak Asian markets, today. The Nifty at the National Stock Exchange fell 57 points, or 1.1 percent, to 5,259.Stock markets in Japan, China, Hong Kong, and South Korea declined between 0.1 percent and 1.6 percent.