Primary Carnitine Deficiency (PCD) is a genetic disorder that affects the body's ability to use fatty acids for energy, specifically in muscle and heart cells. This condition is caused by mutations in the SLC22A5 gene, which encodes for the protein responsible for transporting carnitine into cells. Carnitine plays a crucial role in the transport of fatty acids into mitochondria for beta-oxidation, an essential process for energy production.