A 58-year-old male suffered an acute STEMI of the inferolateral wall while running a marathon...

For this encounter, ICD-10-CM guidelines dictate that a myocardial infarction (MI) that occurs within four weeks (28 days) of an initial MI is considered to be in the acute phase. Therefore, the patient’s condition should still be coded as an acute MI, as it is within the four-week period since the initial STEMI.

B. I21.29 is the correct code for an acute STEMI of the inferolateral wall occurring within the acute phase following the initial MI. The code I21.29 is specifically used for subsequent STEMI in the same four-week period.

Explanation of incorrect options:

A. I22.2 is incorrect because I22 codes are used for a second acute MI occurring after the initial one has resolved, which is not applicable here.

C. I21.19 and D. I21.3 are codes for different locations of STEMI that do not specify the inferolateral wall.

Therefore, the correct answer is B. I21.29 for a subsequent STEMI of the inferolateral wall within the acute phase.