PANCE Blueprint GI and Nutrition (9%)

Phenylketonuria

Patient will present as → a 2-month-old infant with blond hair, blue eyes, and fair skin. He has microcephaly and a progressive impairment of cerebral function. On physical examination you notice an eczematous rash and the patient is vomiting. In addition he has a musty, mousy odor.

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Phenylketonuria (PKU) is an autosomal recessive disorder and inborn error of metabolism involving absent or virtually absent phenylalanine hydroxylase (PAH) enzyme activity.

  • Phenylalanine and its metabolites accumulate in the central nervous system, causing mental retardation and movement disorders.
  • Infants are normal at birth - after a few months, mental retardation is evident.
  • Presents as blond, blue-eyed, with fair skin, mental retardation, eczema, and a musty, mousy body odor of phenylacetic acid.
  • Neonates are screened for PKU 24 to 48 h after birth.

Routine neonatal screening

  • In the US and many developed countries, all neonates are screened for PKU 24 to 48 h after birth with one of several blood tests; abnormal results are confirmed by directly measuring phenylalanine levels.
  • Phenylketonuria is said to be present when plasma phenylalanine levels exceed 20 mg/dl.

Treatment is lifelong dietary phenylalanine restriction.

  • All natural protein contains about 4% phenylalanine. Therefore dietary staples include low-protein natural foods (eg, fruits, vegetables, certain cereals).
  • The sweetener aspartame can act as poisons for people with phenylketonuria.
Question 1
Which of the following is not true regarding phenylketonuria?
A
It is an autosomal dominant disorder
B
It is almost always associated with intellectual disability
Hint:
See A for explantion
C
Incidence in the US is 350 cases per 1 million live births
Hint:
See A for explantion
D
It is due to deficiency of the enzyme phenylalanine hydroxylase
Hint:
See A for explantion
Question 1 Explanation: 
Phenylketonuria is an autosomal recessive disorder. All other statements are correct.
Question 2
The skin findings of a patient with phenylketonuria include all of the following except
A
Fair skin and hair
Hint:
See C for explanation
B
Atopic dermatitis
C
Psoriasis
D
Scleroderma like plaques
Question 2 Explanation: 
Psoriasis is not a skin manifestation of phenylketonuria. Children with PKU tend to have a lighter skin, hair, and eye color than unaffected family members, and some may develop a rash similar to infantile eczema.
Question 3
Classic phenylketonuria is said to be present when plasma phenylalanine levels exceed
A
5 mg/dl
Hint:
See D for explanation
B
10 mg/dl
Hint:
See D for explanation
C
15 mg/dl
Hint:
See D for explanation
D
20 mg/dl
Question 3 Explanation: 
Classic phenylketonuria is said to be present when plasma phenylalanine levels exceed 20 mg/dl.
Question 4
When is PKU testing performed
A
24 weeks gestation
B
24 to 48 h after birth
C
48 to 72 h after birth
D
at the first well child visit
Question 4 Explanation: 
In the US and many developed countries, all neonates are screened for PKU 24 to 48 h after birth. Tandem mass spectrometry is the method of choice. It has a low false-positive rate, and excellent accuracy and precision.
Question 5
How often should the phenylalanine levels of infants undergoing treatment for phenylketonuria be monitored?
A
Weekly
B
Twice a week
Hint:
for neonates
C
Monthly
Hint:
for children older than age 2 up till adulthood.
D
Biweekly
Hint:
for toddlers
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