Correct Answer is: 1 (Case 1: RCA & Case 2: LCX)

In Case of inferior STEMI, it is useful to use the algorithm proposed by Zimetbaum et al. for localizing the lesion (N Engl J Med 2003 Mar 6;348(10):933-40. doi: 10.1056/NEJMra022700.). (see diagram below)

You start simply by looking for two findings: III/II ratio and reciprocal ST-changes in I & aVL. 

Case 1: 

By looking at III/II ratio and ST-changes in I & aVL, first It looks like that ST elevation in III>II. Second, there are notable reciprocal changes in I & aVL (ST depression). This combination will have 90% sensitive and 94% PPV for RCA lesions (see diagram below).

Case 2: 

Again by looking at III/II ratio and ST-changes in I & aVL, it looks that ST elevations in III and II are comparable, and there are no impressive reciprocal changes in I & aVL. This will make it suspicious for LCX lesion.

We then look at chest lead for more confirmatory findings: ST depression in antero-septal leads V1-V3 (equivalent to ST elevation in posterior leads!) and ST elevation in lateral leads (I, aVL, V5, and V6).

Here we can see that there is ST-depression in V1-V3, and ST-elevation in V5, V6. Based on the algorithm, these findings are 96% specific for LCX lesion with a PPV over 91%.

Tip / Take-home message:

In RCA lesions remember 2 Rs: Ratio of (III>II) ST elevation, and Reciprocal changes (depression) in I & aVL. If present, then it is likely RCA lesion.

If not, then look for more ST elevation to confirm your suspicion about LCX (V1-V3 st depression which is posterior elevation, V5-V6, I, aVL are lateral elevations which are usually LCX territories). 

Inferior STEMI localization Algorithm: