You are answering a USMLE-style medical question. Question: A 20-day-old child is brought to the emergency department by her parents. They are concerned about lethargy and easy bruising. Her mother reports that she has been eating less for the past few days. Today she was difficult to arouse. She normally nurses four-five times a day, but ate less yesterday and this morning. The girl was born at 39 weeks gestation via spontaneous vaginal delivery at home with minimal prenatal or postnatal care. This is her first visit to a physician of any type. The family is carefully questioned and child abuse is ruled out. The daughter’s pulse is 97/min, respirations are 35/min, temperature is 35.8°C (96.4°F), and blood pressure is 71/46 mm Hg. Physical examination shows diffuse petechiae and bruising. Which of the following is the most likely cause? Options: A) Group B streptococcus (GBS) infection B) Hepatitis B infection C) Thyroid stimulating hormone deficiency D) Vitamin K deficiency Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.