By Dan Weil The median price for single-family existing homes jumped 15.7% in the first quarter from a year earlier to $368,200. A major element of the inflation gripping the economy is home prices, which just keep rising. The median price for single-family existing homes soared 15.7% in the first quarter from a year ago to $368,200, according to the National Realtors Association. That topped the 14.3% gain for the fourth quarter. Fully 70% of the 185 metropolitan markets tracked by NAR showed double-digit annual price gains in the first quarter. “Prices throughout the country have surged for …