**Core Concept**
Hypertrophy is often confused with hyperplasia in pathology. However, both terms describe changes in cell mass, but they differ in their underlying mechanisms. Hypertrophy involves an increase in cell size, whereas hyperplasia involves an increase in cell number.

**Why the Correct Answer is Right**
Atrophy, on the other hand, is a decrease in cell number, often due to cell death or a reduction in the proliferation of new cells. This can result from various factors, including injury, disease, or disuse of a particular tissue. For instance, in the case of muscle atrophy, the decrease in cell number leads to a reduction in muscle mass and strength. The correct answer, C. Decrease in cell number, accurately describes atrophy.

**Why Each Wrong Option is Incorrect**
**Option A:** Increase in cell number is actually the definition of hyperplasia, not hypertrophy or atrophy.
**Option B:** Increase in cell size is the correct definition of hypertrophy, not atrophy.
**Option D:** Decrease in cell size is not a recognized term in pathology and does not accurately describe atrophy or any other condition.

**Clinical Pearl / High-Yield Fact**
Remember the mnemonic "Hypertrophy is 'Huge' cells, Hyperplasia is 'Many' cells" to differentiate between these two important concepts in pathology. This will help you to quickly recall the definitions and avoid common exam traps.

**✓ Correct Answer: C. Decrease in cell number**