MEXICO CITY -- Efrain Alvarez is the most recent gem over whom the Mexico and U.S. national teams find themselves scrambling.Alvarez is currently on El Tri 's U-17 squad that is preparing for next week's Cuatro Naciones tournament, as well as for next year's World Cup in the category. He has also played for the United States' U-15 team. He has experiences with both youth squads, but that he currently belongs to Mexico's doesn't mean that's where he'll stay.The young LA Galaxy prospect arrived to Mexico's . . .