Pompe disease is a genetic condition and a type of glycogen storage disease, where glycogen accumulates in the body's cells due to a deficiency of the enzyme acid alpha-glucosidase (GAA). GAA normally breaks down complex sugars, and its absence leads to glycogen buildup in lysosomes (cell compartments responsible for recycling substances). This buildup primarily affects organs and tissues like the heart and skeletal muscles, causing them to deteriorate.