The U.S. economy grew at a moderate annual rate of 2.4 percent in the final three months of last year, even better than previously believed, the government said Thursday. But the Commerce Department said the increase in the gross national product, the broadest measure of economic health, was accompanied by a pickup in inflation, reflecting higher food costs and rising import prices. The 2.4 percent increase represented an upward revision from last month, when the government estimated that the GNP had grown at an annual rate of just 2 percent in the October-December quarter. The economy's momentum would have been an even stronger 3.5 percent except for the lingering effects of last summer's drought, which subtracted more than 1 percentage point from growth. An inflation index tied to the GNP rose at a sharp annual rate of 5.3 percent in the October-December quarter.