Untreated PKU patients often have lighter skin, hair, and eyes than their unaffected siblings, even though PKU is caused by a deficiency in phenylalanine hydroxylase. What is the best biochemical explanation?
APhenylpyruvate directly inhibits melanin synthesis in melanocytes
BExcess phenylalanine competes with tyrosine for transport across cell membranes, reducing the tyrosine available for melanin production
CElevated phenylalanine inhibits the enzyme tyrosinase, blocking the first step in melanin synthesis
DThe PAH deficiency also impairs dopamine synthesis, which normally stimulates melanocyte activity
Tyrosine is the precursor to melanin, and its uptake into cells — including melanocytes — uses shared large neutral amino acid (LNAA) transporters. When phenylalanine accumulates to 10–50 times normal levels, it outcompetes tyrosine (and tryptophan) for these transporters, reducing intracellular tyrosine. With less tyrosine available, melanin synthesis declines, producing the characteristically lighter pigmentation seen in untreated patients relative to their unaffected siblings. Option C is the most tempting distractor — tyrosinase is the key melanin enzyme — but the problem is substrate availability through transport competition, not enzyme inhibition.
Question 2 Multiple Choice
In PKU, the primary cause of intellectual disability and neurological damage in infancy and childhood is:
AAccumulation of phenylpyruvate and phenylacetate, which are directly neurotoxic at high concentrations
BDeficiency of tetrahydrobiopterin (BH₄), which is required for neurotransmitter synthesis throughout the brain
CHigh plasma phenylalanine competitively blocking tryptophan and other amino acids from crossing the blood-brain barrier, depleting serotonin and dopamine in the developing brain
DInadequate tyrosine synthesis, causing myelin deficiency in the developing white matter
This is the crucial mechanistic distinction in PKU. The phenylketones (phenylpyruvate, phenylacetate, phenyllactate) cause the characteristic odor but are not the primary drivers of neural damage. It is the phenylalanine itself that floods the large neutral amino acid transporters at the blood-brain barrier, crowding out tryptophan and reducing its uptake into the brain. Less tryptophan means less serotonin synthesis. The developing brain is especially vulnerable because serotonin and dopamine both play critical roles in neural circuit formation. Option A is the most common wrong answer — the phenylketones are the historical namesake of the disease but not the main toxin.
Question 3 True / False
Phenylpyruvate is the primary cause of neurological damage in untreated PKU.
TTrue
FFalse
Answer: False
Phenylpyruvate gives PKU its name (it is the 'phenylketone' excreted in urine) and causes the characteristic musty odor, but it is NOT the primary cause of brain damage. The neurological harm — intellectual disability, seizures, behavioral problems — is caused by high blood phenylalanine itself, which competitively inhibits transport of tryptophan and other large neutral amino acids across the blood-brain barrier. This substrate competition depletes serotonin and dopamine in the developing brain. The distinction matters for treatment: dietary restriction of phenylalanine directly addresses the actual toxic agent.
Question 4 True / False
Early dietary treatment of PKU can prevent intellectual disability because the blood-brain barrier transport problem is reversible and brain development can fully recover even if started months after birth.
TTrue
FFalse
Answer: False
Early detection and treatment prevent brain damage — but the emphasis is on early. The developing brain has critical windows during which high phenylalanine causes irreversible damage to neural circuit formation. Newborn screening programs detect PKU within days of birth precisely to start dietary treatment before symptoms appear. Treatment started after significant brain damage has occurred cannot reverse existing impairment, only prevent further damage. This is why the Guthrie test (now mass spectrometry) on newborn blood spots is one of the highest-value public health screenings in existence.
Question 5 Short Answer
Explain why PKU treatment focuses on restricting dietary phenylalanine rather than simply supplementing tryptophan or serotonin directly.
Think about your answer, then reveal below.
Model answer: The root problem is competitive inhibition of large neutral amino acid (LNAA) transporters by excess phenylalanine. Even if you supplement tryptophan in the diet, elevated phenylalanine will continue to outcompete it for the transporters that cross the blood-brain barrier — supplemented tryptophan would enter the bloodstream but fail to enter the brain in normal amounts. Reducing dietary phenylalanine lowers its plasma concentration, relieving the competitive blockade and allowing normal proportions of all large neutral amino acids to enter the brain. Serotonin itself cannot cross the blood-brain barrier, so peripheral supplementation would be ineffective regardless.
This question tests understanding of the transport mechanism rather than just the metabolic block. The treatment logic flows directly from the biochemistry: the problem is upstream (too much phenylalanine in the blood), and the fix must address the source rather than trying to compensate downstream. It also illustrates why understanding the full mechanism of a disease — not just which enzyme is missing — is necessary for rational therapeutic design.