A 65-year-old woman reports being housebound despite feeling physically healthy. Several years ago, she fell while shopping although she sustained no injuries, the situation was so upsetting that she become extremely nervous when she had to leave her house unaccompanied, for fear that she would have a panic attack. Because she has no children and few friends whom she can ask to accompany her, she is very distressed that she has few opportunities to venture outside her home. What is the most likely diagnosis