A patient presents with painful vesicular rashes limited to the right side of the face and ear that do not cross the midline, accompanied by ipsilateral facial weakness. What is the most likely diagnosis?

Ramsay Hunt syndrome (herpes zoster oticus) is caused by reactivation of varicella-zoster virus (VZV) in the geniculate ganglion of the facial nerve (CN VII). The classic triad is: (1) ipsilateral facial paralysis, (2) otalgia (ear pain), and (3) vesicular rash on the auricle/ear canal that does not cross the midline. The unilateral distribution following a dermatome/nerve distribution is key — herpes zoster never crosses the midline. Trigeminal neuralgia causes severe unilateral facial pain without rash. Guillain-Barre is an ascending polyneuropathy. Bell's palsy causes unilateral facial paralysis but no vesicular rash. Treatment includes antivirals (acyclovir/valacyclovir) and corticosteroids.