I think I would describe it, rather than as a chain, more as a network. In the case of access to medicines, it clearly has to be there, but whether sourced through patented drugs or through generics, there are multiple ways of getting access to medicines at affordable prices, and one or the other has to be there. There will clearly be issues in delivery within Africa, and the infrastructure and educational problems are critical as well.
So I don't think there is a “most fragile” part; in many cases, there are very viable alternatives that can be sought.