**Core Concept**
Electron microscopy is a crucial tool in identifying the morphological changes that occur in cells undergoing irreversible injury. These changes can be indicative of the type of injury and the stage at which the cell has progressed. In this context, the electron microscopy finding indicating irreversible cell injury is a critical diagnostic feature.

**Why the Correct Answer is Right**
The correct answer, **C. Flocculent densities in the mitochondria**, is indicative of irreversible cell injury. Flocculent densities refer to the presence of amorphous, electron-dense material within the mitochondria. This finding is characteristic of mitochondrial damage, which is a critical event in the progression to irreversible cell injury. Mitochondria are essential for energy production in cells, and their dysfunction leads to a cascade of events that ultimately result in cell death. The presence of flocculent densities in the mitochondria suggests that the cell has reached a point of no return and is headed towards apoptosis or necrosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Dilation of ER is more commonly associated with reversible cell injury, where the endoplasmic reticulum (ER) becomes swollen due to increased osmotic pressure within the cell. This is often seen in conditions such as hypoxia or ischemia.

**Option B:** Dissociation of ribosomes from rough ER can occur in both reversible and irreversible cell injury. It is a non-specific finding and does not necessarily indicate the progression to irreversible cell injury.

**Option D:** An intact cell membrane is actually a feature of reversible cell injury, where the cell membrane remains intact, but the cell has undergone significant changes within its organelles.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the presence of flocculent densities in the mitochondria is a critical finding in identifying irreversible cell injury. This can be a key feature in diagnosing conditions such as acute liver failure or myocardial infarction, where cell injury has progressed to a point of no return.

**✓ Correct Answer: C. Flocculent densities in the mitochondria**