WASHINGTON (MarketWatch) - The sale of existing U.S. homes sank 27.2% in July - the biggest one-month drop ever - largely because of the phase-out of a federal tax credit, according to an industry trade group. The National Association of Realtors said existing-home sales fell to a seasonally adjusted annual rate of 3.83 million in July from a revised 5.26 million the month before. Sales of single-family homes fell to the lowest rate in 15 years. Inventories of unsold homes rose 2.5% to 3.98 million in July, representing a 12.5-month supply, the highest level since at least 1999. The median sales price edged up 0.7% in the past year to $182,600 in July, the NAR said.