This drugmaker faces a key test in the next several years.Merck (MRK 0.53%) stock has been a longtime winner for investors, returning 72% in the past five years. A major factor in this pharmaceutical giant's success is its Keytruda immunotherapy drug, recognized as a global standard of care for various cancers.On the other hand, investors must assess some uncertainty regarding the Keytruda platform, which could lose its patent exclusivity by the end of this decade, forcing Merck to find a new growth driver. ...