Can my job refuse to accept a medical excuse signed and authorized by a physician?
Yes. Your physician has no legal authority over your job. That said, if you otherwise qualify for it, you could take unpaid leave (up to 12 weeks) under the Family and Medical Leave Act (FMLA)--basically, if you've been a full-time employee for a year, you most likely qualify, if you need time off to recuperate or for medical treatment, and IF your employer has at least 50 employees). If you are disabled (which means a disability affecting life and lasting at least 6 months) but can still do your job--the same job you were hired for--with "reasonable" accomodations, your employer would have to provide you those accomodations. Reasonable accomodations are changes in procedure or process, or the provision of assistive technology, which is not too expensive or disruptive, such as letting a cashier with leg problems sit instead of stand; providing a larger, magnified computer screen for someone with vision problems; etc.
However, those are your only rights: to leave under FMLA (or a similar state law), or reasonable accomodations for a disabled person. Otherwise, the employer can insist that you be able to show up for work and do your job; and fire you (or anything short of firing: e.g. demote, suspend, etc.) if you can't. The doctor, again, has no authority over your job.