WASHINGTON — Sales of existing homes plunged 17.8 percent in April.

The National Association of Realtors said Thursday that last month’s decline pushed sales down to a seasonally adjusted annual rate of 4.33 million units, the slowest pace since September 2011.

The sales drop was the largest one-month decline since a 22.5 percent fall in July 2010. That tail-off was preceded by the end of a tax credit intended to pull the housing market out of the 2006 collapse.

The median price for a home sold in April was $286,800, which was an increase of 7.4 percent from a year ago. Lawrence Yun, chief economist of the Realtors group, attributed the big jump in the median price to a lack of enough homes for sale, especially for first-time buyers.