Patient: Pharmacists maintain that doctors should not be permitted

Patient: Pharmacists maintain that doctors should not be permitted to sell the medicine that they prescribe because doctors would then be tempted to prescribe unnecessary medicines in order to earn extra income. But pharmacists have a financial interest in having a monopoly on the sale of prescription medicines, so their objection to the sale of medicines by doctors cannot be taken seriously.

The argument is divided into two parts:

1. Pharmacist's conclusion/opinion/position or stand
2. Patient's conclusion/opinion/position

Pharmacist conclusion: Doctors should not be permitted to sell the medicine that they prescribe
Patient conclusion: Pharmacists have a financial interest in having a monopoly on the sale of prescription medicines, so their objection is not warranted.

option B does the job. It says that:

attempting to discredit a position by questioning the motives of the proponents of that position.

here the discredit of position is the pharmacist's position and patient doubts the explanation/motives of the pharmacist.