A 68-year-old woman presents with a 2-year history of insidious memory loss, initially forgetting recent conversations and appointments, progressing to difficulty with language and executive function. Her family reports she remains oriented to person and place but struggles with new learning. Coronal MRI brain shows bilateral mesial temporal lobe atrophy with disproportionate volume loss of the structure marked **A**. Which of the following best explains the cognitive deficit pattern observed in this patient?