## Core Concept
Glycogen storage diseases (GSDs) are a group of inherited disorders that result from defects in enzymes involved in glycogen synthesis or breakdown. These defects lead to abnormal glycogen accumulation or impaired glycogenolysis, affecting various tissues including liver, muscles, and heart. The question focuses on identifying a GSD that spares muscle tissue.

## Why the Correct Answer is Right
The correct answer, **D. GSD Type I (Von Gierke's disease)**, is a disorder caused by a deficiency of glucose-6-phosphatase. This enzyme is crucial for the final steps of gluconeogenesis and glycogenolysis, primarily in the liver and kidneys. Since muscle tissue does not express glucose-6-phosphatase and relies on different pathways for glucose release, GSD Type I primarily affects the liver and kidneys, sparing muscles.

## Why Each Wrong Option is Incorrect
- **Option A: GSD Type II (Pompe disease)** is incorrect because it results from a deficiency of acid alpha-glucosidase (acid maltase), which affects lysosomal glycogen breakdown. This disease impacts muscles, including cardiac and skeletal muscles.
- **Option B: GSD Type III (Cori's disease or Forbes disease)** is incorrect as it involves a deficiency of the debranching enzyme. This condition affects both liver and muscles.
- **Option C: GSD Type V (McArdle's disease)** is incorrect because it results from a deficiency of myophosphorylase, an enzyme crucial for glycogen breakdown in muscles. Thus, it directly affects muscle tissue.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that GSD Type I (Von Gierke's disease) patients often present with hypoglycemia, lactic acidosis, and hepatomegaly but do not exhibit muscle weakness or myopathy, distinguishing it from other GSDs that affect muscles.

## Correct Answer: D. GSD Type I (Von Gierke's disease)