Final answer:
The quickest onset of succinylcholine occurs via the intravenous route, typically within 30 to 60 seconds. It is metabolized primarily by the plasma enzyme pseudocholinesterase.
Step-by-step explanation:
The onset of succinylcholine (often abbreviated as Sux) is the time it takes for the drug to take effect after administration. The quickest onset of action for Sux occurs via intravenous (IV) route, as it is rapidly distributed to the neuromuscular junction and causes muscle paralysis usually within 30 to 60 seconds after IV administration. Metabolism of Sux is primarily through hydrolysis by the enzyme pseudocholinesterase, also known as butyrylcholinesterase, which is present in the plasma. It is important to note that the question also references the metabolism of primaquine, which is metabolized in part by CYP 2D6, with carboxyprimaquine being the primary metabolite found in plasma, thus suggesting a different metabolic pathway than that of succinylcholine.