The Sensex at the Bombay Stock Exchange dropped 257 points, or 1.5 percent–its worst single-day fall in nearly seven weeks–to 17,233, on Thursday, dragged down by a big fall in IT stocks, and falling global markets. The Nifty at the National Stock Exchange lost 71 points, or 1.3 percent, to 5,235.Stock markets in Japan, Hong Kong, South Korea and Singapore lost between 0.5 percent and 2 percent, on global growth concerns.