**Core Concept**
The conversion of lactic acid to glucose is a process known as gluconeogenesis, which involves the synthesis of glucose from non-carbohydrate sources such as lactate, amino acids, and glycerol. This process bypasses several steps of the glycolytic pathway to conserve energy and maintain blood glucose levels.

**Why the Correct Answer is Right**
In gluconeogenesis, the enzyme pyruvate kinase is bypassed, as it catalyzes the conversion of phosphoenolpyruvate (PEP) to pyruvate in glycolysis. However, in gluconeogenesis, PEP is formed from oxaloacetate, and then converted to pyruvate via the enzyme pyruvate carboxylase and phosphoenolpyruvate carboxykinase. The bypass of pyruvate kinase allows gluconeogenesis to avoid the irreversible step of glycolysis.

* **Option A:** Pyruvate dehydrogenase is involved in the conversion of pyruvate to acetyl-CoA, which is not a step in gluconeogenesis.
* **Option B:** Phosphofructokinase-1 is an enzyme involved in glycolysis, and its bypass is not relevant to gluconeogenesis.
* **Option D:** Glucose-6-phosphatase is an enzyme involved in the final step of gluconeogenesis, where glucose-6-phosphate is converted to glucose.

**Clinical Pearl / High-Yield Fact**
Gluconeogenesis is an essential process for maintaining blood glucose levels during fasting or starvation states, and is also important in the liver for regulating blood glucose levels in response to changes in insulin and glucagon levels.

**Correct Answer: D. Glucose-6-phosphatase**