A 2-year-old girl is suspected to have a glycogen storage disorder based on hepatomegaly, muscle weakness, and elevated serum creatine kinase (1200 U/L). Her parents are consanguineous. Which investigation would best differentiate between Type III GSD (Cori disease) and Type V GSD (McArdle disease)?

Explanation

## Differentiating Type III and Type V Glycogen Storage Disorders ### Clinical Distinction | Feature | Type III (Cori) | Type V (McArdle) | | --- | --- | --- | | **Enzyme defect** | Debranching enzyme (glycogen-4-α-glucosidase) | Muscle phosphorylase | | **Tissues affected** | Liver > muscle | Muscle >> liver | | **Hepatomegaly** | Prominent, early | Minimal or absent | | **Muscle involvement** | Mild, late-onset | Severe, early-onset | | **Fasting hypoglycemia** | Present | Absent | | **Exercise intolerance** | Mild | Severe ("second wind" phenomenon) | | **Lactate response to exercise** | Normal increase | **Absent/blunted** | | **Ammonia response to exercise** | Normal increase | **Exaggerated increase** | **Key Point:** The forearm ischemic exercise test is the most specific investigation to differentiate these two disorders based on their metabolic responses to muscle work. ### Why Forearm Ischemic Exercise Test Is Diagnostic The test measures: 1. **Venous lactate** — reflects glycolytic capacity - Type III: Normal lactate rise (debranching enzyme defect does not block glycolysis) - Type V: **Absent/minimal lactate rise** (phosphorylase deficiency blocks glycogen breakdown) 2. **Venous ammonia** — reflects purine nucleotide cycle activity - Type III: Normal ammonia rise - Type V: **Exaggerated ammonia rise** (compensatory purine cycle activation due to energy deficit) **High-Yield:** The **absent lactate + exaggerated ammonia** pattern is pathognomonic for Type V GSD (McArdle disease). ### Why Other Options Are Incorrect **Serum glucose before and after meal:** - Type III: Postprandial hyperglycemia (debranching enzyme defect impairs glycogen breakdown, but glucose absorption from meal is normal) - Type V: Normal glucose response (muscle phosphorylase deficiency does not affect hepatic glucose production) - This test does NOT differentiate the two disorders reliably **Muscle biopsy with histochemistry:** - Both Type III and Type V show glycogen accumulation in muscle - Type V shows more severe glycogen accumulation, but overlap exists - Histochemistry alone cannot reliably differentiate the two - Enzyme assay on muscle tissue would be needed, but this is not the most practical first-line test **Liver enzyme assay for acid α-glucosidase:** - Acid α-glucosidase deficiency causes Type II GSD (Pompe disease), not Type III or V - This is an incorrect enzyme for the differential diagnosis - This option is a distractor based on enzyme testing in GSD **Clinical Pearl:** The "second wind" phenomenon in Type V GSD (McArdle disease) occurs because patients develop increased blood flow and glucose delivery to muscles after a few minutes of exercise, bypassing the phosphorylase defect. This does not occur in Type III because glycolysis can proceed normally from glucose. **Mnemonic — GSD Exercise Test Response: LAM** - **L** — Lactate: absent in Type V, normal in Type III - **A** — Ammonia: exaggerated in Type V, normal in Type III - **M** — McArdle (Type V) shows the pathognomonic LAM pattern