Students block a street as they clash with national guards during a protest against the government in San CristobalMasked youths are once again blocking streets and burning tires in the Venezuelan city of San Cristobal, the epicenter of last year’s massive anti-government protests. The groups are small and the unrest contained, but dissent is rising in this volatile Andean city, a barometer of frustration with nationwide shortages that are putting pressure on the socialist government of Nicolas Maduro. Students, who also accuse the government of corruption and repression but whom Maduro labels “coupsters,” are threatening to unleash larger demonstrations again. More