College Answer

Some excellent responses to this question showed a clear understanding of the pharmacology 
of these agents – the differing mechanisms of action involving both involving nitric oxide. Better 
answers were able to use this to explain the altered vascular specificity.
Toxicity was similarly well prepared for with a good understanding of the role of cyanide in 
SNP and the low rates of toxicity with GTN. This question was best handled in a tabular format 
which minimised omissions.
Some candidates focused on pharmaceutics, indications and side effects which were not 
allocated any marks.
'Compare & contrast' means the similarities; differences & unique features need to be related 
to each other. Several candidates confused ‘nitrous oxide’ with nitric oxide

Discussion

This was not a straightforward "compare and contrast" question which one might be able to answer with one's brainstem. The examiners wanted to specifically see the differences in the mechanism of action and toxicity, which is slightly more novel and thought provoking. Specifically, one would have to dig deep to find differences in mechanism of action, as ostensibly both drugs exert their effect in basically the same way. You'd have to get creative with definitions of "mechanism" and "action". It does not appear as if pharmacokinetic maters would have attracted any marks, but it was tempting to include them because they attracted marks in previous versions of this question.