**Question:** Type VI glycogen storage disease is due to the deficiency of -

---

**Core Concept:**
Glycogen storage diseases (GSDs) are a group of rare disorders that affect the way the body stores and uses glucose. They are caused by deficiencies in enzymes involved in glycogen synthesis, degradation, or remodeling. In Type VI GSD, the deficiency is in the enzyme phosphoglucomutase-1 (PGM1).

**Why the Correct Answer is Right:**
Type VI GSD is a result of deficiency in phosphoglucomutase-1 (PGM1), which is an enzyme involved in the glycogen degradation pathway. This enzyme catalyzes the conversion of glucose-6-phosphate to fructose-6-phosphate during the breakdown of glycogen into glucose monomers. Without sufficient PGM1 activity, the glycogen breakdown process is impaired, leading to the accumulation of glycogen in the liver, muscles, and other tissues.

**Why Each Wrong Option is Incorrect:**

A. This option is incorrect because the deficiency in this disease is due to the deficiency of phosphofructokinase-1 (PFK-1), which is involved in the glycolysis pathway, not glycogen degradation.

B. This option is incorrect as it refers to the deficiency of fructose-1,6-bisphosphatase, which is involved in the gluconeogenesis pathway, not glycogen degradation.

C. This option is incorrect since the deficiency in this disease is attributed to the deficiency of glycogen phosphorylase, an enzyme involved in glycogen breakdown, not glycogen synthesis.

D. This option is incorrect as it refers to the deficiency of glucose-6-phosphatase, an enzyme involved in glucose homeostasis and gluconeogenesis, not glycogen degradation.

**Clinical Pearl / High-Yield Fact:**
Type VI GSD is a rare disorder, with a prevalence of approximately 1 in 1,000,000 births. The condition is characterized by elevated liver enzymes, hepatomegaly, and muscle weakness. Treatment involves dietary management, including a low-carbohydrate diet and supplemental glucose to prevent hypoglycemia.

---

**Correct Answer:** C. Glycogen phosphorylase deficiency

In conclusion, Type VI Glycogen Storage Disease is caused by the deficiency of an enzyme called glycogen phosphorylase, which is responsible for breaking down glycogen in the liver and muscles. This deficiency leads to the accumulation of glycogen in these tissues, causing the clinical presentation described above.