You are answering a USMLE-style medical question. Question: A 3-month-old girl is brought to the physician because of a productive cough for 5 days. Over the past month, she has had several episodes of watery stools. She is exclusively breastfed every 3–4 hours for 15–20 minutes. She was delivered vaginally at 38 weeks' gestation at home and has not yet been evaluated by a physician. The mother reports that her child is not gaining weight. The mother had no prenatal care. The infant is at the 5th percentile for height and weight. The infant has not received any immunizations. Her temperature is 38.5°C (101.3°F), pulse is 155/min, respirations are 45/min, and blood pressure is 88/50 mm Hg. Oral examination shows white plaques covering the tongue and the palate. Rales are heard bilaterally on cardiopulmonary examination. Cervical and inguinal lymphadenopathy is present. Which of the following is most likely to confirm the diagnosis? Options: A) Karyotyping of infant's chromosomes B) DNA test for CFTR mutation C) Fluorescent treponemal antibody absorption test D) Polymerase chain reaction for viral genes Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.