Shares of the medical-device maker DexCom (DXCM 5.57%) are down by 37% this year. The diabetes-focused healthcare company disappointed investors with its second-quarter results, leading to a massive drop in its stock price overnight. Though the issues DexCom has faced this year can mainly be classified as "short-term headwinds" -- including rebate-related issues -- there are more serious problems for the company's long-term prospects.On the other hand, DexCom has attractive opportunities that could allow it ...