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.
osmosis Osmosis
Picmonic
Phenylketonuria

Phenylketonuria or PKU is a metabolic disorder caused by decreased levels of the enzyme phenylalanine hydroxylase, or alternatively decreased levels of its tetrahydrobiopterin cofactor. Phenylketonuria is inherited in an autosomal recessive fashion. For affected patients, tyrosine becomes an essential amino acid. Treatment requires avoiding phenylalanine intake from the diet, which is found in products containing aspartame, such as Nutra Sweet. Clinically, patients’ urine will have a musty or mousy body odor. They can also exhibit growth retardation, seizures, mental retardation, and skin hypopigmentation.

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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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