**Core Concept**
Glycolysis is a critical metabolic pathway that converts glucose into pyruvate, producing energy in the form of ATP and NADH. The defect in this pathway can lead to various clinical manifestations, including anemia, due to impaired energy production and increased oxidative stress.

**Why the Correct Answer is Right**
The patient's symptoms suggest a severe anemia, which is likely caused by a defect in the glycolytic pathway. The absence of Heinz bodies and abnormally low lactate production in red blood cells point towards a deficiency in an enzyme that catalyzes a step in glycolysis. Pyruvate kinase is a key enzyme in the glycolytic pathway that catalyzes the conversion of phosphoenolpyruvate to pyruvate. A deficiency in pyruvate kinase would lead to impaired energy production and increased oxidative stress in red blood cells, resulting in anemia.

**Why Each Wrong Option is Incorrect**
**Option A:** Glucose-6-phosphate dehydrogenase (G6PD) deficiency would lead to Heinz body formation due to oxidative stress, which is not observed in this patient.
**Option B:** Phosphofructokinase (PFK) deficiency would also lead to impaired glycolysis, but it would not specifically result in low lactate production.
**Option C:** Hexokinase deficiency would impair glucose uptake in red blood cells, but it would not directly lead to low lactate production.

**Clinical Pearl / High-Yield Fact**
Pyruvate kinase deficiency is a rare genetic disorder that affects the glycolytic pathway, leading to impaired energy production and increased oxidative stress in red blood cells. It is characterized by severe anemia, jaundice, and hemolytic anemia.

**Correct Answer:** C. Pyruvate kinase.