**Core Concept**
In myocardial infarction, the early light microscopic changes occur due to the ischemic damage to cardiac muscle cells. This damage leads to the disruption of the normal architecture of the cardiac tissue.

**Why the Correct Answer is Right**
The waviness of fibres is an early light microscopic change observed in myocardial infarction. This occurs due to the contraction band necrosis, where the actin and myosin filaments within the cardiac muscle cells undergo contraction in the absence of ATP, leading to the disruption of the normal fibre architecture. This change is observable under light microscopy as early as 30 minutes after the onset of ischemia.

**Why Each Wrong Option is Incorrect**
**Option A is incorrect** because while waviness of fibres is an early change, it is not the only change observed in myocardial infarction. Other changes such as fibre necrosis and round cell infiltration occur later.

**Option B is incorrect** because fibre necrosis is a later change in myocardial infarction, occurring several hours after the onset of ischemia.

**Option C is incorrect** because round cell infiltration is a late change in myocardial infarction, occurring several days after the onset of ischemia, as part of the repair process.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the early changes in myocardial infarction can be subtle and may not always be visible under light microscopy. A thorough understanding of the pathophysiology of myocardial infarction is crucial for accurate diagnosis.

**Correct Answer Line**
✓ Correct Answer: A. Waviness of fibres