Based on the passage, where in the United States would a physician encounter the most cases of SAD?

We cannot see the passage, but SAD stands for Seasonal Affective Disorder. SAD is a type of depressive disorder related to lack of light, and therefore changes with the seasons. It is a severe version of the "winter blues"

In the winter months, where the days are shorter, individuals who suffer from SAD suffer from mood changes. They get depressed, have a reduction in energy levels, and withdraw socially. They may also gain weight. This causes problems with relationships, work, and school.

Since SAD is linked to light, SAD is likely more common in the Northern areas of the United States, where winter days are shorter.