I guess it depends on what you mean by "challenging". If you define "challenging" as meaning that each and every patient you encounter will present with a problem where you have to think deeply in order to treat it, then no, I'd say that medicine isn't challenging--at least, not in that way. Every profession--even car racing, I'd imagine--has such a thing as routine and even monotony. Of course, you certainly will encounter patients who do have challenging problems that require a lot of logical thought on your part. This is certainly the case for me as a dentist, although admittedly, the diversity of pathologies that physicians deal with is greater than ours, and therein lies the bulk of the added challenge of being a physician as opposed to any other type of health care provider. That and the fact that you often deal with diseases that are life-threatening.
What makes being a doctor (physician, dentist, podiatrist, optometrist, and so on) a challenging career, truly, is the fact that every patient you treat deserves the same level of thoroughness and diligence on your part. Why? Well, they're putting their health in your hands, for one. Also, because where health care is concerned, the devil is often in the details. Every patient you see may turn out to be that one patient whose problem seems like a no-brainer but may have some small, obscure, but immensely significant positive finding in their health history that you might miss.
As a doctor, you have to do everything for everybody from A to Z. There is no profession that requires the same level of diligence and thoroughness. That's why being a doctor is so challenging.