Lens has a respiratory quotient of:

**Core Concept**
The respiratory quotient (RQ) is a measure of the ratio of carbon dioxide production to oxygen consumption by a cell or tissue. It is an important indicator of the type of metabolic pathways being utilized by a particular organ or tissue.

**Why the Correct Answer is Right**
The lens in the eye is a highly metabolic tissue that requires a constant supply of energy to maintain its transparency and refractive index. The lens primarily utilizes glucose as its energy source, and its high metabolic rate is fueled by glycolysis, a process in which glucose is broken down to produce energy. As a result, the lens has a high respiratory quotient, typically around 1.0, indicating that it is primarily metabolizing glucose. The high RQ of the lens is also a reflection of its high rate of glycolysis, which is essential for maintaining its unique physiological properties.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately reflect the metabolic pathway utilized by the lens. The lens is not primarily a fatty acid-metabolizing tissue.
**Option B:** This option is incorrect because it does not accurately reflect the respiratory quotient of the lens. The lens has a high RQ, indicating that it is primarily metabolizing glucose.
**Option C:** This option is incorrect because it does not accurately reflect the metabolic pathway utilized by the lens. The lens is not primarily a protein-metabolizing tissue.

**Clinical Pearl / High-Yield Fact**
The lens is a highly metabolic tissue that requires a constant supply of energy to maintain its transparency and refractive index. Its high respiratory quotient is a reflection of its high rate of glycolysis, which is essential for maintaining its unique physiological properties.

**Correct Answer: D. 1.0**