A 15 year old female patient presents with pain in the calf muscles on exercise. Routine investigation reveals burgundy-colored urine (sample obtained after exercise). Histopathology of muscle shows deposits of glycogen in an excess amount. What is the diagnosis?
First, the key symptoms: muscle pain during exercise and dark urine. The dark urine after exercise makes me think of myoglobinuria, which can occur when muscle breakdown happens. But the presence of excess glycogen in the muscle suggests a glycogen storage disease.
Glycogen storage diseases (GSDs) are a group of disorders where glycogen metabolism is impaired. The ones affecting skeletal muscle would be type V (McArdle disease) and maybe type VII (Tarui disease). McArdle disease is caused by a deficiency of muscle glycogen phosphorylase, which is needed to break down glycogen for energy during exercise. Without that enzyme, the muscle can't produce glucose, leading to pain, cramps, and sometimes rhabdomyolysis. The rhabdomyolysis would release myoglobin, causing the dark urine. The burgundy color is a classic sign of myoglobinuria.
Looking at the options, if the correct answer is McArdle disease (GSD type V), then the other options would be other GSDs. Let's consider why others are wrong. Type II (Pompe disease) is lysosomal acid alpha-glucosidase deficiency, causing glycogen accumulation in lysosomes, but it's more systemic and affects infants more. Type III (Cori disease) involves debranching enzyme deficiency, but that's more about liver and muscle, but I'm not sure if it presents with exercise intolerance. Type VI (Hers disease) is liver-specific, so probably not relevant here.
The clinical pearl here is that McArdle disease presents with exercise intolerance and myoglobinuria, and the key histopath is excess glycogen in muscles. The student should remember that muscle glycogen phosphorylase deficiency is the main issue here, leading to the symptoms described.
**Core Concept**
This question tests knowledge of glycogen storage diseases (GSDs) affecting skeletal muscle. The clinical scenario points to a **myopathy** with **exercise-induced rhabdomyolysis**, characterized by **myoglobinuria (burgundy urine)** and **muscle glycogen accumulation**. The key is linking enzyme deficiency to clinical presentation.
**Why the Correct Answer is Right**
The diagnosis is **McArdle disease (GSD type V)**, caused by **muscle glycogen phosphorylase deficiency**. During exercise, muscles cannot break down glycogen to glucose, leading to energy starvation, pain, and cramps. Rhabdomyolysis releases **myoglobin**, causing **myoglobinuria** (burgundy-colored urine). Histopathology shows **excess glycogen** due to impaired utilization, not synthesis. This is a classic "second wind" phenomenon—rest relieves pain, but resuming exercise exacerbates it.
**Why Each Wrong Option is Incorrect**
**Option A:** *Pompe disease (GSD type II)* involves **lysosomal alpha-glucosidase deficiency**, causing glycogen accumulation in **lysosomes** (not cytoplasmic glycogen). It typically presents with **infantile hypotonia**