How to get Nose to run doctests from my Markdown documentation?

[Skip Montanaro]

I routinely include doctests as a source of test cases in my nose runs, but
I want to also coax it to check the examples in my Markdown files. Is there
a way to do this? If I explicitly give a Markdown file on the command line,
nose complains:

Unable to load tests from file ...

Am I perhaps missing some sort of nose-markdown plugin which would
magically make this work?

Thx,

Skip