**Core Concept**
Glycolysis and gluconeogenesis are two metabolic pathways that involve the breakdown and synthesis of glucose, respectively. Both pathways share several common enzymes, but there are key differences in their regulation and enzyme composition.

**Why the Correct Answer is Right**
Glucose-6-phosphatase is a unique enzyme that is not shared between glycolysis and gluconeogenesis. Instead, it is a key enzyme in gluconeogenesis that catalyzes the final step of glucose production, converting glucose-6-phosphate to glucose. This enzyme is crucial for glucose homeostasis and is primarily found in the liver and kidneys. In contrast, glycolysis and gluconeogenesis have several common enzymes, including phosphoglycerate kinase, phosphoglycerate mutase, and aldolase.

**Why Each Wrong Option is Incorrect**
**Option A:** Aldolase is a common enzyme in both glycolysis and gluconeogenesis, catalyzing the conversion of fructose-1,6-bisphosphate to glyceraldehyde-3-phosphate and dihydroxyacetone phosphate.
**Option C:** Phosphoglycerate mutase is also a shared enzyme between glycolysis and gluconeogenesis, facilitating the conversion of 3-phosphoglycerate to 2-phosphoglycerate.
**Option D:** Phosphoglycerate kinase is another common enzyme in both pathways, catalyzing the conversion of 1,3-bisphosphoglycerate to 3-phosphoglycerate, generating ATP in the process.

**Clinical Pearl / High-Yield Fact**
Remember that glucose-6-phosphatase is a key regulatory enzyme in gluconeogenesis, and its deficiency can lead to glycogen storage disease type I (GSD-I), characterized by hypoglycemia, lactic acidemia, and hepatomegaly.

✓ Correct Answer: B. Glucose-6-phosphatase