A 38-year-old woman presents with acute-onset right-sided facial droop over 24 hours, inability to close her right eye, loss of taste on the anterior two-thirds of the tongue, and hyperacusis. Examination confirms lower motor neuron facial palsy with loss of forehead wrinkling. MRI shows abnormal enhancement of the facial nerve at the geniculate ganglion, labyrinthine segment, and tympanic segment as marked by **A** in the diagram. No vesicles are seen in the ear canal or palate. Which of the following is the most likely diagnosis and underlying pathophysiology?