A dose equal to [X amount] is equal to *what* those patients [suffering the most extreme pain] should take.
The above sentence is valid. Now we add "even" to modify "those patients". Here we are adding in some attitude and saying "EVEN THOSE PATIENTS", implying that patients suffering extreme pain take a LARGE dose...and a dose of X amount of another product is as powerful as a large dose.
Add "even":
A dose equal to [X amount] is equal to *what* even those patients [suffering the most extreme pain] should take.
Change "equal to" to "as much as"
A dose equal to [X amount] is as much as *what* even those patients [suffering the most extreme pain] should take.
Remove *what* since it's implicit, understood, implied.
A dose equal to [X amount] is as much as even those patients [suffering the most extreme pain] should take.
All of these sentences are valid and (A) is just the final result here.