**Question:** Which of the following is caused by defective transport of tryptophan:

A. Sjögren-Larsson syndrome
B. Carnitine palmitoyltransferase II deficiency
C. Maple syrup urine disease
D. Tyrosinemia

**Core Concept:** Tryptophan is an essential amino acid required for the synthesis of serotonin, melatonin, and niacin (vitamin B3). The transport of tryptophan across cellular membranes is regulated by the presence of other amino acids, mainly phenylalanine and tyrosine. Defective transport of tryptophan can lead to imbalances in neurotransmitter synthesis and subsequent neurological manifestations.

**Why the Correct Answer is Right:** Carnitine palmitoyltransferase II deficiency (CPT II deficiency) is a disorder of fatty acid beta-oxidation caused by mutations in the CPT2 gene, leading to a deficiency in carnitine palmitoyltransferase II enzyme. This results in accumulation of long-chain fatty acids, which can cause neuropathies and encephalopathies.

**Why Each Wrong Option is Incorrect:**

A. Sjögren-Larsson syndrome (SLS) is a genetic disorder caused by mutations in the ALDH3A2 gene, leading to deficiency of fatty aldehyde dehydrogenase enzyme. SLS presents with neurological symptoms, but it is not caused by defective tryptophan transport.

B. Tyrosinemia is a group of genetic disorders characterized by defects in tyrosine catabolism, not tryptophan transport. Tyrosinemia is caused by mutations in different genes involved in tyrosine catabolism, leading to abnormal accumulation of tyrosine and its derivatives.

D. Maple syrup urine disease (MSUD) is a disorder of branched-chain amino acid catabolism, caused by mutations in BCKDH genes. MSUD involves the deficiency of branched-chain α-keto acid dehydrogenase complex, leading to the accumulation of branched-chain keto acids and their derivatives.

**Clinical Pearl:** In medical exams, understanding the regulation of amino acid transport and their roles in neurotransmitter synthesis is crucial, as amino acid transport defects can lead to neurological symptoms. Maintaining a balance of amino acids is essential for normal brain function, and understanding the mechanisms of this balance can help differentiate between various neurological disorders.