Due to my studies I often find that I do not have enough time to properly complete a workout, therefore I end up prioritising what exercises I do over others. Although this is not the best way to utilise the time spent at the gym (ideally you should have enough time to complete every aspect of your routine) sometimes it is unavoidable. Even if you have been so lucky as to not have had to do this, you will in the conceivable future. So in order to prepare you for this challenge I will explain to you not only what to prioritise but also how the selection process works.
So even if you know that you will have a condensed session or you find that you are short of time mid-session it does not matter just follow these simple steps. First of all you need to identify what your major compound lift is in your days program. If you find that you do not have one then I would refer you to my series on creating a proper training split parts 1, 2 and 3. This compound lift could take the form of a squat, bench press or even something a little less common like a Romanian Deadlift. Now that you have found it make sure that that exercise is the one that is completed first and with the greatest focus. This will allow you to train the most muscles in the shortest amount of time. Next I would select an exercise that was not heavily targeted by the first compound motion. So if your initial exercise was a deadlift then do something such as a bench-supported dumbbell row- this will put some emphasis on muscles such as you lattissimus dorsi, teres major/minor and your rhomboids. To give a little time update, this would take fifteen to twenty minutes. Thirdly you should give this spot up to another isolation exercise- this time it should help and assist with the compound lift. Keeping with the deadlift theme a farmers carry or suitcase carry would be appropriate to assist with trapezius, forearm and trunk stability development.
Really If I were to sum all of this up it is mainly about training as many muscles as possible in the short space of time you have available. Despite the rush do not forget what your intended goal is. Is it to improve a specific lift? Is it to become more athletic? Whatever it is always keep it in mind as each session regardless of length or intensity will bring you closer to your aim. And even if you are only at the gym for forty-five minutes and can only do four exercises it does not matter. Consistency must be the end game not over-compensating week after week for small mistakes- work with what you have not with what you have not.