Step 1: Understanding the Concept:
Class 1 anti-arrhythmics are split into 1A, 1B and 1C based on how strongly and how quickly they bind and release sodium channels, and the question wants the one drug among four that does not belong to the 1C group.
Step 2: Key Concept or Approach:
Recall that Class 1C drugs bind sodium channels tightly and dissociate slowly, causing marked slowing of conduction, while Class 1B drugs like lidocaine and its relatives dissociate quickly and have a much weaker effect on conduction.
Step 3: Detailed Explanation:
Propafenone, flecainide and moricizine are all textbook Class 1C agents, each producing strong, long-lasting sodium channel blockade that slows cardiac conduction significantly.
Mexiletine, on the other hand, is structurally and pharmacologically related to lidocaine, and it dissociates from sodium channels rapidly, giving it the milder conduction effect typical of Class 1B rather than Class 1C.
Because three of the four drugs fit the 1C profile and mexiletine does not, mexiletine stands out as the exception.
Step 4: Final Answer:
Mexiletine is NOT a Class 1C anti-arrhythmic drug; it belongs to Class 1B.