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Chapter 30: Nitrogen: Amino Acid Metabolism

You are examining a pediatric patient brought to you by his parents because they are disturbed by his progressive mood changes and apparent hightened anxiety. Physical examination shows pellegra-like skin eruptions and signs of cerebellar ataxia that include unsteady gait and uncoordinated eye movements. Serum and urinalysis show significant aminoacidemia and aciduria. These signs and symptoms are most likely due to which of the following disorders?

A. alkaptonuria

B. glycine decarboxylase deficiency (GCC deficiency)

C. Hartnup disorder

D. maple syrup urine disease

E. PKU

Answer C: Hartnup disorder is caused by a defect in neutral amino acid transport in the apical brush border membranes of the small intestine and in kidney proximal tubules. The lack of intestinal tryptophan transport is responsible for most if not all clinical phenotypes of Hartnup disorder. The pellagra-like skin rash seen on sun-exposed areas of skin in Hartnup disorder patients is most likely the result of nicotinamide deficiency due to a lack of tryptophan, which is a precursor for its synthesis. Symptoms of Hartnup disorder may begin in infancy or early childhood, but sometimes they begin as late as early adulthood. Symptoms may be triggered by sunlight, fever, drugs, or emotional or physical stress. Most symptoms occur sporadically and are caused by a deficiency of niacin. When Hartnup disorder manifests during infancy, the symptoms can be variable in clinical presentation. These symptoms include failure to thrive, photosensitivity, intermittent ataxia, nystagmus, and tremor.

You are examining an infant brought to your office due to the onset of seizures. The parents report that their baby has also exhibited a progressive lack of energy, feeding difficulties, hypotonia, abnormal jerking movements, and difficulty breathing. These symptoms are suggestive of a defect in the processes of neurotransmission and thus, are most likely due to which of the following disorders?

A. alkaptonuria

B. glycine decarboxylase deficiency (GCC deficiency)

C. Hartnup disorder

D. maple syrup urine disease

E. PKU

Answer B: Nonketotic hyperglycinemia (NKH) is an autosomal recessive inborn error of glycine metabolism due to deficiencies in the H, P, or T proteins of the glycine decarboxylase complex. NKH, also known as glycine encephalopathy, is characterized by severe mental retardation that is due to highly elevated levels of glycine in the CNS. Infants generally present with severe lethargy, seizures, and respiratory depression requiring mechanical ventilation. A diagnosis of NKH associated with these symptoms is made secondary to elevated ...

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