Review Question - QID 109354

A 7-day-old girl presents with her parents to the pediatrician for her first well-child visit. Her mother reports that the patient is being exclusively breastfed. The patient’s mother is concerned about a rash at the nape of the patient’s neck. Her parents are not sure when the rash first developed and the patient’s mother is concerned that the patient is having an allergic reaction to something in her breastmilk. The patient’s mother reports that the patient is urinating 6-8 times per day and stools 3 times per day. The patient’s mother reports that the patient’s stools are loose, dark yellow, and “seedy”. She denies ever seeing any streaks of blood in the patient’s diaper. Her temperature is 97.9°F (36.6°C), blood pressure is 60/46 mmHg, pulse is 145/min, and respirations are 30/min. On physical exam, the patient appears well-developed and well-nourished. Her anterior fontanelle is soft and flat. The patient has the physical exam finding seen in Figure A. The lesion blanches with pressure. Which of the following is the most likely cause of this patient's physical exam finding?