NEW YORK — Just how nervous are investors about Greece? All it took to derail a day of stock market gains was a headline saying that the country was preparing to leave the euro, an outcome many analysts expect to happen eventually. Major indexes were higher for most of the day after the National Association of Realtors reported that home prices surged 10 percent over the past year, the biggest gain in six years.

Then, with less than an hour of trading left, news hit that Greece’s former prime minister said the country was considering dropping the euro. Investors have been anticipating that Greece could make a messy exit from the euro this year, but the prospect that it could be imminent gave the market a jolt.