**Core Concept**
Glycolysis is a metabolic pathway that breaks down glucose to pyruvate, generating energy in the form of ATP. The pathway consists of 10 steps, of which 3 are irreversible, requiring energy input to proceed.

**Why the Correct Answer is Right**
One of the key irreversible steps in glycolysis is catalyzed by the enzyme pyruvate kinase, which converts phosphoenolpyruvate (PEP) to pyruvate, producing one molecule of ATP. However, the enzyme phosphoglucose isomerase (PGI) catalyzes the conversion of glucose-6-phosphate (G6P) to fructose-6-phosphate (F6P), which is a reversible step.

**Why Each Wrong Option is Incorrect**
* **Option A:** Phosphofructokinase (PFK) is an enzyme that catalyzes the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate, an irreversible step in glycolysis.
* **Option B:** Aldolase is an enzyme that catalyzes the conversion of fructose-1,6-bisphosphate to glyceraldehyde-3-phosphate and dihydroxyacetone phosphate, a reversible step in glycolysis.
* **Option C:** Triosephosphate isomerase (TPI) is an enzyme that catalyzes the conversion of glyceraldehyde-3-phosphate to dihydroxyacetone phosphate, a reversible step in glycolysis.
* **Option D:** Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is an enzyme that catalyzes the conversion of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate, an irreversible step in glycolysis.

**Clinical Pearl / High-Yield Fact**
Glycolysis is a critical pathway for energy production in cells, particularly in tissues with high energy demands such as the brain and muscle. Inhibitors of glycolytic enzymes, such as 2-deoxyglucose, can disrupt cellular energy metabolism and have been explored as potential therapeutic agents.

**Correct Answer:** B. Phosphoglucose isomerase.