Even if you "know" what is going on, there should always be a differential. Especially as a student. There's always that not rare case of what appears to be bronchiolitis but turns out to be pneumonia, or the rare case that turns out to be heart disease. For rashes, there's always a few different things it could possibly be, in order from most to less likely. As a student, you always want to say, the most likely cause is "X" because of reasons "A, B and C, though reasons D, E go against it", less likely it could be "Y" because of reasons "DE, but reasons A,C,D go against it", etc. My plan would be to "obtain lab x and based on that do x, or begin treatment xyz for presumed X and if unsuccessful then try abc".
No one likes someone who appears bored by what they do. Clinic may appear not challenging because people aren't constantly being intubated or on ECMO, but that doesn't mean that you should be foregoing a differential or your critical thinking skills.
Also when you give your reasons and a broad differential even for common conditions 1) it forces you to critically analyze your thinking, and you'll realize when you try to say something out loud if it doesn't make sense, 2) it gives opportunities for discussion where the resident or the attending can tell you the finer points of the diagnosis or management of what may seem to be common problems, 3) it shows that you're interested and invested in learning, which is much more important than saying that you're interested in a topic field, 4) you'll avoid getting "locked in" to a specific diagnosis and when the course doesn't go as expected you'll find it much easier to step back and figure out what you might have missed and where to go, and 5) you'll often teach your residents and attendings! Ask why we do the things we do! You'll be surprised the stuff you know. And most people have less of an ego about this stuff than you'd think. For example, I just learned something from a medical student yesterday about the management of a certain skin condition. I told him that his plan was "wrong" and I would do it this way, but apparently the student had just came from a dermatology rotation and the dermatologists were doing something very different than most pediatricians. And he brought the evidence. We spent a lot of time looking at the literature and I learned a lot. You can bet that's going to go into that students evaluation in an absolutely glowing manner!
Hey man I don't know what the issue originally was since you deleted your post, but my advice is to purchase a peds er textbook that goes through each symptom and gives you a wide differential with the disease that you have to rule out and cannot miss and then present those in differential so you can you are thinking about these things and have ruled them out. The big problem with just latching on to a common diagnosis first is that you sometimes don't get these important rule out going through your head and you miss them