Q. A 35-year-old female presents to the emergency department with a rash that she reports developed over the past few days. The patient states that the rash is extremely itchy and has spread to multiple areas of her body, including her arms, legs, and torso (see image). She also reports experiencing abdominal pain, diarrhea, and weight loss over the past several months. Her past medical history is significant for iron deficiency anemia and chronic headaches. On physical examination, her temperature is 36.5 degrees Celsius (98.6 degrees Fahrenheit), her blood pressure is 135/80 mm Hg, heart rate 80/min, and respiratory rate is 18/min. You note numerous fluid-filled blisters and raised, red, itchy patches of skin on the extensor surfaces of her elbows and knees. Her abdomen also demonstrates widely distributed reddish-purple bumps on an erythematous base. You perform a skin biopsy, which reveals subepidermal blister formation with neutrophils located at the tips of the dermal papillae and perivascular inflammatory cell infiltrate. Further laboratory testing shows that the patient also has elevated levels of specific antibodies. Which of the following is the most likely offending agent?