Question

Q-T elongation is seen in which drug?

• A. Quinidine
• B. Amiodarone
• C. Magnesium Sulfate
• D. Lignocaine

Answer 1

The Correct Option is A

Step 1: Identify the electrophysiologic clue. Q-T lengthening reflects delayed ventricular repolarisation, and the dangerous rhythm it predisposes to is the twisting-of-the-points tachycardia ($Torsades$ $de$ $Pointes$).
Step 2: Map each option to its Vaughan-Williams class. Quinidine = Class IA, amiodarone = Class III, lignocaine = Class IB, and magnesium sulphate is a therapeutic ion, not an antiarrhythmic class drug.
Step 3: Class IA agents block both sodium and the $I_{Kr}$ potassium channel; the potassium block is what stretches repolarisation and gives the longest Q-T. Quinidine is the prototype and the single most frequently cited offender.
Step 4: Eliminate the rest. Magnesium is the antidote you push for Torsades, lignocaine trims the action potential rather than prolonging it, and amiodarone rarely triggers Torsades despite a long Q-T because its repolarisation effect is uniform.
\[\boxed{Quinidine}\]