The stimulus focuses on explaining a particular phenomenon: the fact that licensed physicians do not prescribe herbs as drugs. The author starts by summarizing some people's argument that the reason for this phenomenon is that the medical effectiveness of herbs is doubtful.

But the author then suggests another explanation: drugs cannot be offered for sale unless they have regulatory approval, and to get regulatory approval requires around 200 million dollars. But such money can only be expected for a drug that is patented, and herbs themselves cannot be patented. Thus, the real reason doctors do not prescribe herbs, the author concludes, is regulatory and financial restrictions that make it impossible for doctors to prescribe herbs.

The author questions other people's explanation for why physicians do not prescribe herbs. She does this by providing an alternative explanation for this phenomenon: under the current regulations, herbs and their medicinal uses cannot be patented, which makes it impossible to offer them for sale. Thus, the author concludes that the reason physicians do not prescribe herbs is that they are unable to, under the current system of regulations — not necessarily that physicians are concerned that herbs are not effective, as the other people suggest.

There are some big assumptions in this argument — for instance, that physicians can only prescribe a drug if it is offered for sale. But our job for this Method of Reasoning question is just to describe how the author goes about making her argument, even if that argument has some flaws. To summarize, the author undermines one proposed explanation of a phenomenon by offering an alternative explanation for it.