I'm very grateful to you all for your kindness and great advice. As a matter of fact, Dorothy, I am taking generic Sinemet. Perhaps that's something to consider.

The MDS I'm with now is actually the doctor I went to for a second opinion - the original general neurologist diagnosed PD almost immediately based on my medical history, a clear brain MRI, and my neurological exam. He prescribed Azilect, which I took for three weeks with no apparent effect. He then switched me to Stalevo 100, which I took 4 times a day for 4 weeks, again with no apparent effect other than a wild ride with nausea and vomiting. He then switched me to Sinemet SR, which didn't make much difference either in symptom control or side-effects (still sick as an ugly dog). His opinion was (I'm paraphrasing), "Yeah, that's weird. Probably means MSA." I sought a second opinion.

The MDS I'm seeing now ordered blood tests to rule out Wilson's, verified the brain MRI was clear of "space-occupying defects", ordered a SPECT scan, and repeated the medical history and neuro exam (although his neuro exam seemed a bit more comprehensive). His conclusion at this point is probable idiopathic PD, but wants to review the SPECT scan. He said there's an outside chance it could be something like corticobasal degeneration, but warned me not to get carried away with that thought. His view was that the original drug challenge was so haphazard as to be useless in supporting a diagnosis. He believes the original challenge didn't raise my dopamine levels high enough to be conclusive, and that the nausea/vomiting were only complicating things. He prescribed regular carbidopa/levodopa on a slowly graduated schedule, with extra carbidopa in the mornings to control the nausea. He also prescribed an antinausea pill if things got too bad, but it made me so sleepy I couldn't get through the day without ending up under my desk.

I think my next steps are to see what the SPECT scan says and to perhaps try the non-generic Sinemet. I've been operating under the belief that my lack of response to dopamine means P+; I'm relieved to hear that it may not be so cut-and-dried, but the engineer in me wants answers. But like my buddy Mick says, "you can't always get what you want." :-)

Thanks again for your help - please don't stop if there's more to offer. I'll let you know what I learn.