**Core Concept**
Muscle hypertrophy is a physiological adaptation characterized by an increase in the size of muscle fibers, leading to enhanced muscle strength and endurance. This process involves changes in the muscle cell structure, including the growth of myofibrils and the synthesis of new proteins.

**Why the Correct Answer is Right**
In hypertrophy, there is a significant increase in the expression of genes involved in protein synthesis, such as the mTOR (mechanistic target of rapamycin) pathway. This leads to an increase in the translation of mRNA into proteins, resulting in the growth of new muscle tissue. Additionally, hypertrophy is associated with an increase in the number of myonuclei, which are essential for muscle growth and repair.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because hypertrophy is not primarily characterized by an increase in muscle cell number, but rather an increase in muscle fiber size.
**Option B:** This option is incorrect because atrophy, not hypertrophy, is associated with a decrease in muscle protein synthesis and an increase in muscle cell death.
**Option C:** This option is incorrect because muscle hypertrophy is not solely dependent on the presence of growth hormone, although it may play a role in regulating muscle growth.

**Clinical Pearl / High-Yield Fact**
A key factor in muscle hypertrophy is the presence of resistance training, which stimulates the mTOR pathway and promotes muscle growth.

**Correct Answer: D. Muscle protein synthesis is increased, leading to an increase in muscle fiber size.