Preheat the oven to 350F. In a large bowl, combine the ground beef, stuffing mix, egg, and milk; mix well. Place in a 9" x 5" loaf pan that has been coated with nonstick cooking spray. Bake for 45 minutes then pour off any excess liquid. Spread the ketchup evenly over the top and bake for 10 more minutes, or until no pink remains in the meat.