a 51 year-old man has persistent substernal burning epigastric pain. cbc reveals a wbc count of 6,600 (normal 4,00-11,00)/microliter. his hgb is 11.3 (normal 13-17.3) g/dl, hct 32.1 (normal 38-52)%, mcv 75 (normal 80-98) fl, and platelet count 168,000 (normal 140,000-400,000) /microliter. the most cost-effective initial test to perform at this point in the management of this patient is a(an): group of answer choices