**Core Concept**
The underlying principle being tested is the distinction between reversible and irreversible cell injury at the ultrastructural level, particularly focusing on the characteristics visible under electron microscopy. This involves understanding the **morphological changes** that occur in cells as they progress from reversible to irreversible injury.

**Why the Correct Answer is Right**
In irreversible cell injury, one of the hallmark features observed under electron microscopy is the presence of **amorphous densities** within the mitochondria, indicating severe mitochondrial damage. Other features may include **chromatin condensation**, **plasma membrane blebbing**, and **loss of microvilli**, but the specific correct answer choice is not provided.

**Why Each Wrong Option is Incorrect**
**Option A:** This choice is incorrect because it does not specifically relate to the characteristic ultrastructural changes seen in irreversible cell injury.
**Option B:** Similarly, this option does not accurately describe the features of irreversible injury visible under electron microscopy.
**Option C:** This choice is also incorrect as it fails to represent the characteristic morphological alterations associated with irreversible cell damage.
**Option D:** Without the specific details of each option, it's clear that only one can accurately describe the ultrastructural features of irreversible injury.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **mitochondrial changes**, including the formation of amorphous densities, are critical indicators of irreversible cell injury. Recognizing these features under electron microscopy is essential for diagnosing cell death.

**Correct Answer:** D. Amorphous densities in mitochondria