**Core Concept**
The question is testing the understanding of epimerism and the metabolism of monosaccharides in the human body. Specifically, it is asking about the formation of an L-isomer of a monosaccharide, which involves the epimerization of a monosaccharide at a specific carbon atom.

**Why the Correct Answer is Right**
The correct answer involves the formation of L-glucose from D-glucose through the action of the enzyme aldose reductase in the polyol pathway. This enzyme catalyzes the reduction of D-glucose to L-glucose, which is a crucial step in the regulation of glucose metabolism in the human body. The polyol pathway is activated in conditions of hyperglycemia, where D-glucose is converted to L-glucose and then to fructose, which is then metabolized further.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to the question and does not provide a correct answer to the formation of an L-isomer of a monosaccharide.
* **Option B:** This option is incorrect because it does not specify the formation of an L-isomer of a monosaccharide and is not related to the polyol pathway.
* **Option C:** This option is incorrect because it is not a correct answer to the formation of an L-isomer of a monosaccharide and is not related to the polyol pathway.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the polyol pathway is activated in conditions of hyperglycemia, leading to the formation of L-glucose from D-glucose. This pathway is particularly relevant in the context of diabetes mellitus, where hyperglycemia is a hallmark of the disease.

**Correct Answer:** C. L-glucose