A 55-year-old woman presents with C/O of vaginal discomfo. It first staed 6 months ago and has fluctuated in intensity, most recently causing itching and slight pain when she has intercourse with her husband. She otherwise has been well. She has been postmenopausal for 3 years. Her older sister recommended an estrogen cream, which she has used consistently for >3 months without any change in symptoms. On examination the introital mucosa and labia minora are whitish-pink, with abnormal wrinkling and a small fissure on the right labia minora. Which of the following is the most likely diagnosis?