This question checks whether you can read the electrical story hidden in three raw ECG traces and connect it to what is physically happening in the heart. Let's walk through each option pairing.
Option (A): X-(i); Y-(iii); Z-(ii): X is an evenly paced trace with one clean P wave for every QRS, exactly what a healthy conduction system produces, so X fits the normal condition. Y shows disturbed timing between atrial and ventricular beats, a classic sign of AV block. Z shows fast, sawtoothed oscillations, the textbook look of atrial flutter. Every pairing checks out.
Option (B): X-(i); Y-(ii); Z-(iii): this keeps X as normal, which is right, but swaps Y and Z. Y does not show the rapid sawtooth firing rate of flutter, and Z does not show the delayed conduction pattern of AV block, so this swap is wrong.
Option (C): X-(ii); Y-(i); Z-(iii): this calls X atrial flutter and Y normal, but X lacks the fast repetitive flutter waves, and Y is not the cleanest, most regular trace of the three, so this pairing does not fit the evidence.
Option (D): X-(ii); Y-(iii); Z-(i): this calls Z normal, but Z is the trace with the fast sawtooth pattern, the opposite of what a normal rhythm looks like, so this is wrong.
Reading the three traces by their conduction pattern (clean and regular, delayed or blocked, or fast and sawtoothed) gives X-(i), Y-(iii), Z-(ii), option (A).
Let's summarize:
A normal ECG has one P wave before every QRS at a steady rate.
AV block shows delayed or dropped conduction between atria and ventricles.
Atrial flutter shows a fast, repeating sawtooth pattern from rapid atrial firing.
So the correct match is X-(i), Y-(iii), Z-(ii), option (A).
