Week 15

Question 1

What happens to amino acids during metabolism?

Answer 1

The amino group is removed, and the remaining carbon skeleton enters central metabolic pathways.

Question 2

What are carbon skeletons converted into?

Answer 2

Pyruvate, acetyl-CoA, acetoacetyl-CoA, or TCA cycle intermediates.

Question 3

Why is carbon skeleton metabolism important?

Answer 3

It supports energy production, gluconeogenesis, and ketogenesis.

Question 4

Which pathways supply intermediates for nonessential amino acid synthesis?

Answer 4

Glycolysis and the TCA cycle.

Question 5

Which amino acids are branched-chain amino acids?

Answer 5

Leucine, isoleucine, valine.

Question 6

Why are BCAAs unique among amino acids?

Answer 6

They are primarily metabolized in skeletal muscle, not the liver.

Question 7

What enzyme complex is critical for BCAA metabolism?

Answer 7

Branched-chain α-ketoacid dehydrogenase (BCKD).

Question 8

What happens if BCAA metabolism is defective?

Answer 8

Toxic accumulation of BCAAs and their α-keto acids, especially in the brain.

Question 9

What causes inborn errors of amino acid metabolism?

Answer 9

Enzyme deficiencies in amino acid metabolic pathways.

Question 10

Why are these disorders dangerous?

Answer 10

Toxic metabolites accumulate and interfere with brain development.

Question 11

Why is early diagnosis critical?

Answer 11

Early dietary intervention can prevent irreversible neurologic damage.

Question 12

What is Maple Syrup Urine Disease?

Answer 12

An autosomal recessive disorder caused by BCKD deficiency.

Question 13

Which amino acids accumulate in MSUD?

Answer 13

Leucine, isoleucine, valine and their α-keto acids.

Question 14

Why is leucine especially dangerous in MSUD?

Answer 14

It is highly neurotoxic and can cause coma.

Question 15

Classic clinical features of MSUD?

Answer 15

Poor feeding, vomiting, ketoacidosis, seizures, coma, intellectual disability.

Question 16

What causes the maple-syrup odor in MSUD?

Answer 16

Accumulation of isoleucine metabolites.

Question 17

How is MSUD treated?

Answer 17

Lifelong diet restricting BCAAs with specialized amino-acid formulas.

Question 18

Which MSUD variant responds to vitamin therapy?

Answer 18

Thiamine (vitamin B1)–dependent MSUD.

Question 19

What enzyme is deficient in classic PKU?

Answer 19

Phenylalanine hydroxylase (PAH).

Question 20

What accumulates in PKU?

Answer 20

Phenylalanine and phenylketones.

Question 21

Why does PKU cause brain damage?

Answer 21

Phenylalanine is neurotoxic and reduces neurotransmitter synthesis.

Question 22

Why is tyrosine deficient in PKU?

Answer 22

Phenylalanine cannot be converted to tyrosine.

Question 23

Key clinical features of untreated PKU?

Answer 23

Intellectual disability, seizures, microcephaly, hypopigmentation.

Question 24

Why do PKU patients have fair skin and light hair?

Answer 24

Tyrosine deficiency reduces melanin synthesis.

Question 25

What cofactor is required for PAH?

Answer 25

Tetrahydrobiopterin (BH4).

Question 26

What is nonclassic (BH4-deficient) PKU?

Answer 26

Impaired BH4 synthesis or regeneration, affecting dopamine and serotonin synthesis.

Question 27

Why doesn’t dietary Phe restriction fully treat BH4 deficiency?

Answer 27

Neurotransmitter synthesis remains impaired.

Question 28

When should PKU screening be done?

Answer 28

24–48 hours after birth following protein feeding.

Question 29

Main treatment for classic PKU?

Answer 29

Low-phenylalanine diet + tyrosine supplementation.

Question 30

Why must aspartame be avoided in PKU?

Answer 30

Aspartame metabolism produces phenylalanine.

Question 31

What is albinism?

Answer 31

A disorder of tyrosine metabolism causing reduced melanin production.

Question 32

Mode of inheritance of most albinism forms?

Answer 32

Autosomal recessive.

Question 33

Major clinical features of albinism?

Answer 33

Hypopigmented skin, hair, eyes; visual impairment.

Question 34

Why are albino patients sensitive to sunlight?

Answer 34

Lack of melanin protection.

Question 35

Increased long-term risk in albinism?

Answer 35

Skin cancer.

Question 36

What is homocystinuria?

Answer 36

A disorder of homocysteine metabolism.

Question 37

Most common enzyme deficiency in homocystinuria?

Answer 37

Cystathionine β-synthase.

Question 38

Biochemical findings in homocystinuria?

Answer 38

↑ Homocysteine, ↑ methionine, ↓ cysteine.

Question 39

Why are connective tissues affected?

Answer 39

Homocysteine disrupts disulfide bonds in fibrillin.

Question 40

Classic clinical features of homocystinuria?

Answer 40

Lens dislocation, osteoporosis, skeletal abnormalities, thrombosis.

Question 41

Why is homocystinuria life-threatening?

Answer 41

High risk of premature vascular disease and thrombosis.

Question 42

Which vitamins lower homocysteine levels?

Answer 42

Vitamin B6, B12, and folate.

Question 43

Dietary management of homocystinuria?

Answer 43

Methionine restriction and vitamin supplementation.

Question 44

How do enzyme deficiencies cause toxicity?

Answer 44

Substrates accumulate and damage the nervous system.

Question 45

Why is newborn screening essential?

Answer 45

Symptoms may not be present at birth.

Question 46

How do dietary interventions prevent complications?

Answer 46

They limit toxic metabolites while allowing normal growth.