i recently bought usmleRx..it has first aid incorporated into it..thats what makes it so great! u can save time actually opening up first aid and instead have the page open up right in front of u as u answer the questions..butt im not sure if the difficulty level is equal to usmle world..does anyone know? I want the best prep ever..if usmlerx is close to the difficulty level of usmleworld..then i would rather stick to usmlerx..since it has an additional feature..that being first aid!