RIO DE JANEIRO, Aug 19 (Reuters) - Mariana Pajon sparked noisy celebrations from a yellow army of Colombian fans as she retained her Olympic BMX title on Friday. The 24-year-old from Medellin, who became a national treasure after winning Colombia's second-ever Olympic gold medal in London, did not put a wheel out of place over the jumps and round the curves of Rio's X Park track. She led from the start gate and was in total control as she led Alise Post of the United States and Venezuela's Stefany Hernandez across the line. ... More...