You are answering a USMLE-style medical question. Question: A 27-year-old college student presents to his physician for constant fatigue. The patient states that for the past several weeks he has felt oddly fatigued and has struggled to complete his work. He is a foreign exchange student and is concerned he will fail out of his academic program. He has experienced some occasional nausea and vomiting which has caused him to lose weight. The patient is a first year college student, drinks alcohol regularly, smokes marijuana, and has unprotected sex with multiple partners. His temperature is 99.5°F (37.5°C), blood pressure is 127/68 mmHg, pulse is 90/min, respirations are 17/min, and oxygen saturation is 98% on room air. Laboratory values are ordered as seen below. Serum: Na+: 139 mEq/L Cl-: 100 mEq/L K+: 4.3 mEq/L HCO3-: 25 mEq/L BUN: 20 mg/dL Glucose: 99 mg/dL Creatinine: 1.1 mg/dL Ca2+: 10.2 mg/dL Total bilirubin: 2.0 mg/dL AST: 85 U/L ALT: 92 U/L Urine Color: Amber/brown Marijuana: Positive Amphetamines: Positive Cocaine: Positive The patient refuses an examination of his genitals. Which of the following is the best diagnostic test? Options: A) CAGE questions B) Uretheral swab and culture C) Viral core antibody D) Viral surface antibody Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.