**Core Concept**
The question is testing the knowledge of the protein content in various types of cellular membranes. The correct answer is related to the high protein content in a specific type of membrane due to its functional importance.

**Why the Correct Answer is Right**
Myelin sheath, which is the correct answer, is the highest in protein content per gram of tissue. This is because myelin sheath is composed of a high concentration of lipids and proteins, particularly lipoproteins, which are essential for its structure and function. The myelin sheath is responsible for insulating nerve fibers and facilitating the transmission of nerve impulses. The high protein content in myelin sheath is crucial for its proper functioning.

**Why Each Wrong Option is Incorrect**
* **Option A:** Plasma membrane, while rich in proteins, does not have the highest protein content per gram of tissue compared to myelin sheath.
* **Option B:** Endoplasmic reticulum, a type of organelle involved in protein synthesis and transport, does not have a high protein content compared to myelin sheath.
* **Option D:** Mitochondrial membrane, while essential for energy production, does not have the highest protein content per gram of tissue compared to myelin sheath.

**Clinical Pearl / High-Yield Fact**
Myelin sheath is composed of a high concentration of lipids and proteins, particularly lipoproteins, which are essential for its structure and function. This is a critical aspect of neurology and is relevant to conditions such as multiple sclerosis, where demyelination occurs.

**Correct Answer:** C. Myelin sheath.