Metabolic diseases are a group of disorders in which the normal biochemical processes of the body are disrupted due to defects in enzymes, hormones, transport proteins, or metabolic pathways. These defects interfere with the conversion of nutrients such as carbohydrates, proteins, and fats into energy or essential cellular components. Many metabolic diseases are inherited and result from mutations in specific genes, while others may develop because of environmental factors, hormonal imbalance, or nutritional deficiencies. In human genetics, inherited metabolic diseases are commonly known as inborn errors of metabolism and are usually transmitted in an autosomal recessive, autosomal dominant, or X-linked manner. Early diagnosis and proper management are essential to prevent severe complications and improve quality of life. Phenylketonuria (PKU) is one of the best-known inherited metabolic disorders. It is caused by mutations in the PAH gene, leading to deficiency of the enzyme phenylalanine hydroxylase. As a result, phenylalanine accumulates in the blood and tissues, causing intellectual disability, developmental delay, seizures, behavioral problems, and light pigmentation if untreated. Newborn screening enables early detection, and treatment consists of a low-phenylalanine diet and regular monitoring of blood phenylalanine levels. Galactosemia is another inherited metabolic disease caused by deficiency of enzymes involved in galactose metabolism, most commonly galactose-1-phosphate uridyltransferase (GALT). Infants develop vomiting, jaundice, liver enlargement, cataracts, poor feeding, and failure to thrive after consuming milk. If untreated, severe liver damage, intellectual disability, and infections may occur. Management includes lifelong elimination of lactose and galactose from