**Core Concept**
Reye's syndrome is a rare but serious condition that affects the liver and brain, characterized by an encephalopathy and fatty degeneration of the liver. The histological changes in Reye's syndrome are distinct and crucial for diagnosis.

**Why the Correct Answer is Right**
The correct answer, glycogen depletion, is a hallmark of Reye's syndrome. The liver cells in affected individuals show a significant depletion of glycogen stores, which is a result of the mitochondrial dysfunction and increased fatty acid oxidation. This depletion of glycogen stores leads to the accumulation of fat droplets in the liver cells, causing steatosis. The underlying mechanism involves the inhibition of mitochondrial pyruvate dehydrogenase, leading to an increase in NADH and a subsequent decrease in glycogen synthesis.

The liver cells in Reye's syndrome also show an increase in mitochondrial size and a decrease in cristae, indicating mitochondrial dysfunction. However, the key histological finding is the depletion of glycogen stores, which distinguishes Reye's syndrome from other conditions that cause liver damage.

**Why Each Wrong Option is Incorrect**
**Option A:** Budding and branching of mitochondria is a feature of mitochondrial myopathies, such as Kearns-Sayre syndrome, but not Reye's syndrome.

**Option B:** Swelling of endoplasmic reticulum is a feature of various conditions, including viral hepatitis and drug-induced liver injury, but is not specific to Reye's syndrome.

**Option C:** Para-nuclear micro-dense deposits are characteristic of viral hepatitis, particularly hepatitis B and C, but are not a feature of Reye's syndrome.

**Clinical Pearl / High-Yield Fact**
Reye's syndrome is a rare but serious condition that typically affects children and adolescents who have been given aspirin during a viral infection. It is essential to avoid aspirin in children with viral infections to prevent this condition.

**✓ Correct Answer: D. Glycogen depletion**