Activists seeking to strike down a ban on medical marijuana outlets in Los Angeles saw their challenge qualify for the ballot, dealing a setback to the city's latest attempt at a crackdown. Backers of medical marijuana dispensaries needed 27,425 valid signatures to force a referendum on a law that prohibits the sale of cannabis but allows groups of three people or fewer to cultivate and share the drug. The Los Angeles City Council voted 14-0 in July to ban medical marijuana shops. Foes said the proliferation of dispensaries had gotten out of control.