**Core Concept**
Lysosomes are membrane-bound organelles responsible for cellular digestion and recycling of macromolecules. They contain hydrolytic enzymes that break down proteins, lipids, and carbohydrates into smaller molecules, which can then be reused or excreted by the cell.

**Why the Correct Answer is Right**
Residual bodies are formed when lysosomes fuse with other cellular components, such as phagosomes or autophagosomes, containing damaged or dysfunctional organelles, proteins, or other cellular debris. The resulting structure is a residual body, which is essentially a lysosome containing the accumulated waste material. The enzymes within the lysosome continue to break down the contents of the residual body, allowing the cell to recycle the resulting products.

**Why Each Wrong Option is Incorrect**
**Option A:** Mitochondria are the site of cellular respiration and energy production, not cellular digestion or waste disposal.
**Option B:** The nucleus contains the cell's genetic material and is not involved in the breakdown of cellular components.
**Option C:** The Golgi apparatus is responsible for protein modification, sorting, and packaging, but it is not involved in the degradation of cellular waste.

**Clinical Pearl / High-Yield Fact**
Lysosomes play a critical role in maintaining cellular homeostasis and preventing cellular damage. Defects in lysosomal function have been implicated in various diseases, including lysosomal storage disorders, such as Tay-Sachs disease and Pompe disease.

**✓ Correct Answer: D. Lysosomes**