herpes zoster

A 27-year-old pregnant female presents with a painful rash to the right side of her face that started 4 days ago. She is G1P0 and currently 26 weeks pregnant with reported routine prenatal care. She had been doing well until 4 days prior when she began to experience tingling and pain to her right cheek, followed by the development of small, painful, erythematous bumps to the area. Her VS are normal. On exam, she has a vesicular rash across her right cheek in the V2 cranial nerve distribution. What is the most likely cause of this woman’s rash?