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First Aid for the USMLE Step 1 > Biochemistry > Flashcards

Biochemistry Flashcards

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USMLE® Step 1 flashcards
1
Q

Vitamin A (retinol)

Function?

A

Antioxidant;

Constituent of visual pigments (retinal);

Essential for normal differentiation of epithelial cells into specialized tissue (pancreatic cells, mucus-secreting cells);

Prevents squamous metaplasia.

Used to treat measles and AML, subtype M3.

Retinol is vitamin A, so think retin-A (used topically for wrinkles and acne).
Found in liver and leafy vegetables.

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2
Q

Vitamin A (retinol)

Deficiency?

A

Night blindness (nyctalopia);

Dry, scaly skin (xerosis cutis);

Alopecia;

Corneal degeneration (keratomalacia);

Bitot spots on conjunctiva;

Immune suppression.

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3
Q

Vitamin A (retinol)

Excess?

A

Acute toxicity - nausea, vomiting, vertigo, and blurred vision

Chronic toxicity - alopecia, dry skin (e.g. scaliness), hepatic toxicity and enlargement, arthralgias, and pseudotumor cerebri

Teratogenic (cleft palate, cardiac abnormalities), so a (-) pregnancy test and
reliable contraception are needed before isotretinoin (vitamin A derivative) is prescribed for severe acne.

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4
Q

Vitamin B1 (thiamine)

Function?

A

In thiamine pyrophosphate (TPP), a cofactor for several dehydrogenase enzyme reactions:

  • Pyruvate dehydrogenase (links glycolysis to TCA cycle)
  • ƒƒ α-ketoglutarate dehydrogenase (TCA cycle)
  • ƒƒ Transketolase (HMP shunt)
  • ƒƒ Branched-chain ketoacid dehydrogenase

Think ATP: α-ketoglutarate dehydrogenase, Transketolase, and Pyruvate dehydrogenase. Spell beriberi as Ber1Ber1 to remember vitamin B1.

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5
Q

Vitamin B1 (thiamine)

Deficiency?

A

Impaired glucose breakdown Ž–> ATP depletion worsened by glucose infusion; highly aerobic tissues (e.g., brain, heart) are affected first.

Wernicke-Korsakoff syndrome and beriberi. Seen in malnutrition and alcoholism (2° to malnutrition and malabsorption).

Diagnosis made by increase in RBC transketolase activity following vitamin B1 administration.

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6
Q

Wernicke-Korsakoff syndrome

A

Wernicke-Korsakoff syndrome—confusion, ophthalmoplegia, ataxia (classic triad) + confabulation, personality change, memory loss (permanent). Damage to medial dorsal nucleus of thalamus, mammillary bodies.

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7
Q

Beriberi

A

Dry beriberi—polyneuritis, symmetrical muscle wasting.
Wet beriberi—high-output cardiac failure (dilated cardiomyopathy), edema.

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8
Q

Vitamin B2 (riboflavin)

Function?

A

Component of flavins FAD and FMN, used as cofactors in redox reactions, e.g., the succinate dehydrogenase reaction in the TCA cycle.

FAD and FMN are derived from riboFlavin (B2 = 2 ATP).

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9
Q

Vitamin B2 (riboflavin)

Deficiency?

A

Cheilosis (inflammation of lips, scaling and fissures at the corners of the mouth), Corneal vascularization.

The 2 C’s of B2.

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10
Q

Vitamin B2 (niacin)

Function?

A

Constituent of NAD+, NADP+ (used in redox reactions).

Derived from tryptophan.

Synthesis requires vitamins B2 and B6.

Used to treat dyslipidemia;

Lowers levels of VLDL and raises levels of HDL.

NAD derived from Niacin (B3 = 3 ATP).

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11
Q

Vitamin B3 (niacin)

Deficiency?

A

Glossitis.

Severe deficiency leads to pellagra, which can be caused by Hartnup disease (decrease tryptophan absorption), malignant carcinoid syndrome (increase tryptophan metabolism), and isoniazid (decrease vitamin B6).

Symptoms of pellagra: Diarrhea, Dementia (also hallucinations), Dermatitis (C3/C4 dermatome circumferential “broad collar” rash [Casal necklace], hyperpigmentation of sun exposed limbs)

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12
Q

Vitamin B3 (niacin)

Excess?

A

Facial flushing (induced by prostaglandin, not histamine; can avoid by taking aspirin with niacin), hyperglycemia, hyperuricemia.

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13
Q

Vitamin B5 (pantothenic acid)

Function?

A

Essential component of coenzyme A (CoA, a cofactor for acyl transfers) and fatty acid synthase.

B5 is “pento”thenate.

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14
Q

Vitamin B5 (pantothenate)

Deficiency?

A

Dermatitis, enteritis, alopecia, adrenal insufficiency.

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15
Q

Vitamin B6 (pyridoxine)

Function?

A

Converted to pyridoxal phosphate (PLP), a cofactor used in transamination (e.g., ALT and AST), decarboxylation reactions, glycogen phosphorylase.

Synthesis of cystathionine, heme, niacin, histamine, and neurotransmitters including serotonin, epinephrine, norepinephrine (NE), dopamine, and GABA.

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16
Q

Vitamin B6 (pyridoxine)

Deficiency?

A

Convulsions, hyperirritability, peripheral neuropathy (deficiency inducible by isoniazid and oral contraceptives), sideroblastic anemias due to impaired hemoglobin synthesis and iron excess.

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17
Q

Vitamin B7 (biotin)

Function?

A

Cofactor for carboxylation enzymes (which add a 1-carbon group):
ƒƒ Pyruvate carboxylase: pyruvate (3C)Ž –> oxaloacetate (4C)
ƒƒ Acetyl-CoA carboxylase: acetyl-CoA (2C) –>Ž malonyl-CoA (3C)
ƒƒ Propionyl-CoA carboxylase: propionyl-CoA (3C) Ž–> methylmalonyl-CoA (4C)

“Avidin in egg whites avidly binds biotin.”

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18
Q

Vitamin B7 (biotin)

Deficiency?

A

Relatively rare. Dermatitis, alopecia, enteritis.

Caused by antibiotic use or excessive ingestion of raw egg whites.

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19
Q

Vitamin B9 (folate)

Function?

A

Converted to tetrahydrofolate (THF), a coenzyme for 1-carbon transfer/methylation reactions.

Important for the synthesis of nitrogenous bases in DNA and RNA.

Found in leafy green vegetables.

Absorbed in jejunum.

Folate from foliage.

Small reserve pool stored primarily in the liver.

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20
Q

Vitamin B9 (folate)

Deficiency?

A

Macrocytic, megaloblastic anemia;

hypersegmented polymorphonuclear cells (PMNs);

glossitis;

no neurologic symptoms (as opposed to vitamin B12 deficiency).

Labs: increased homocysteine, normal methylmalonic acid.

Most common vitamin deficiency in the United States.

Seen in alcoholism and pregnancy.

Deficiency can be caused by several drugs (e.g., phenytoin, sulfonamides, methotrexate).

Supplemental maternal folic acid in early pregnancy decreases risk of neural tube defects

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21
Q

Vitamin B12 (cobalamin)

Function?

A

Cofactor for homocysteine methyltransferase (transfers CH3 groups as methylcobalamin) and methylmalonyl-CoA mutase.
Found in animal products.
Synthesized only by microorganisms.

Very large reserve pool (several years) stored primarily in the liver.

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22
Q

Vitamin B12 (cobalmin)

Deficiency?

A

Macrocytic, megaloblastic anemia;

hypersegmented PMNs;

paresthesias,

and subacute combined degeneration (degeneration of dorsal columns, lateral
corticospinal tracts, and spinocerebellar tracts) due to abnormal myelin.

Associated with increased serum homocysteine and methylmalonic
acid levels. Prolonged deficiency Ž–> irreversible nerve damage.

Deficiency is usually caused by insufficient intake (e.g., veganism),
malabsorption (e.g., sprue, enteritis, Diphyllobothrium latum), lack of intrinsic factor (pernicious anemia, gastric bypass surgery), or absence of terminal ileum (Crohn disease).

Anti-intrinsic factor antibodies diagnostic for pernicious anemia.

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23
Q

Vitamin C (asorbic acid)

Function?

A

Antioxidant.
Also facilitates iron absorption by reducing it to Fe2+ state.
Necessary for hydroxylation of proline and lysine in collagen synthesis.
Necessary for dopamine β-hydroxylase, which converts dopamine to NE.
Found in fruits and vegetables.
Pronounce “absorbic” acid.
Ancillary treatment for methemoglobinemia by reducing Fe3+ to Fe2+.

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24
Q

Vitamin C (asorbic acid)

Deficiency?

A

Scurvy—swollen gums, bruising, petechiae, hemarthrosis, anemia, poor wound healing, perifollicular and subperiosteal hemorrhages, “corkscrew” hair.

Weakened immune response.

Vitamin C deficiency causes sCurvy due to a Collagen synthesis defect.

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25
Q

Vitamin C (asorbic acid)

Excess?

A

Nausea, vomiting, diarrhea, fatigue, calcium oxalate nephrolithiasis.

Can increase risk of iron toxicity in predisposed individuals (e.g., those with transfusions, hereditary hemochromatosis).

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26
Q

Vitamin D

Function?

A 
D2 = ergocalciferol—ingested from plants.
D3 = cholecalciferol—consumed in milk, formed in sun-exposed skin (stratum basale).
25-OH D3 = storage form.
1,25-(OH)2 D3 (calcitriol) = active form.
Drinking milk (fortified with vitamin D) is good for bones.

Increase intestinal absorption of calcium and
phosphate, increase bone mineralization.

27
Q

Vitamin D

Deficiency?

A

Rickets in children (bone pain and deformity), osteomalacia in adults (bone pain and muscle weakness),

hypocalcemic tetany.

Breastfed infants should receive oral vitamin D.

Deficiency is exacerbated by low sun exposure, pigmented skin, prematurity.

Rickets: medial angulation and outward bowing of femurs and tibia (fenu varum)

28
Q

Vitamin D

Excess?

A

Hypercalcemia, hypercalciuria, loss of appetite, stupor.

Seen in granulomatous disease ( increased activation of vitamin D by epithelioid macrophages).

29
Q

Vitamin E (tocopherol/tocotrienol)

Function?

A

Antioxidant (protects erythrocytes and membranes from free radical damage).

E is for Erythrocytes.

Can enhance anticoagulant effects of warfarin.

30
Q

Vitamin E

Deficiency?

A

Hemolytic anemia,

acanthocytosis,

muscle weakness,

posterior column and spinocerebellar tract demyelination.

Neurological presentation may appear similar to vitamin B12 deficiency, but without megaloblastic anemia, hypersegmented neutrophils, or increase serum methylmalonic acid levels.

31
Q

Vitamin K

Function?

A

Cofactor for the γ-carboxylation of glutamic acid residues on various proteins required for blood clotting.

Synthesized by intestinal flora.

K is for Koagulation.

Necessary for the activation of clotting factors II, VII, IX, X, and proteins C and S.

Warfarin—vitamin K antagonist.

32
Q

Vitamin K

Deficiency?

A

Neonatal hemorrhage with increase PT and increase aPTT but normal bleeding time (neonates have sterile intestines and are unable to synthesize vitamin K).

Can also occur after prolonged use of broad-spectrum antibiotics.

Not in breast milk;

neonates are given vitamin K injection at birth to prevent hemorrhage disease of the newborn

33
Q

Zinc

Function?

A

Mineral essential for the activity of 100+ enzymes.

Important in the formation of zinc fingers (transcription factor motif).

34
Q

Zinc

Deficiency?

A

Delayed wound healing, hypogonadism, decrease adult hair (axillary, facial, pubic), dysgeusia, anosmia, acrodermatitis enteropathica (well-demarcated, scaly plaques in intertriginous area).

May predispose to alcoholic cirrhosis.

35
Q

Kwashiorkor

A

Protein malnutrition resulting in skin lesions, edema due to decreased plasma oncotic pressure, liver malfunction (fatty change due to decrease apolipoprotein synthesis).

Clinical picture is small child with swollen belly.

Kwashiorkor results from a protein-deficient MEAL:
Malnutrition
Edema
Anemia
Liver (fatty)

36
Q

Marasmus

A

Total calorie malnutrition resulting in tissue and muscle wasting, loss of subcutaneous fat, and variable edema.

Marasmus results in Muscle wasting

37
Q

Autosomal recessive diseases

A

Albinism, ARPKD, cystic fibrosis, glycogen storage diseases, hemochromatosis, Kartagener syndrome, mucopolysaccharidoses (except
Hunter syndrome), phenylketonuria, sickle cell anemia, sphingolipidoses (except Fabry disease), thalassemias, Wilson disease.

38
Q

Cystic fibrosis

Genetics?

A

Autosomal recessive;

defect in CFTR gene on chromosome 7;

commonly a deletion of Phe508.

Most common lethal genetic disease in Caucasian population.

39
Q

Cystic fibrosis

Pathophysiology?

A

CFTR encodes an ATP-gated Cl- channel that secretes Cl- in lungs and GI tract, and reabsorbs Cl- in sweat glands.

Most common mutation –>Ž misfolded protein –>Ž protein retained in RER and not transported to cell membrane, causing decrease Cl- (and H2O) secretion; 

Increase intracellular Cl- results in compensatory increase Na+ reabsorption via epithelial Na+ channels Ž–> increase H2O reabsorption –> abnormally thick mucus secreted into lungs and GI tract.

Na+ reabsorption also causes more negative transepithelial potential difference.

40
Q

Cystic fibrosis

Diagnosis?

A

Increase Cl- concentration (>60 mEq/L) in sweat is diagnostic.

Can present with contraction alkalosis and hypokalemia (ECF effects analogous to a patient taking a loop diuretic) because of ECF H2O/Na+ losses and concomitant renal K+/H+ wasting.

Increase immunoreactive trypsinogen (newborn screening)

41
Q

Cystic fibrosis

Complications?

A

Recurrent pulmonary infections (e.g., Pseudomonas), chronic bronchitis and bronchiectasis –>Ž reticulonodular pattern on CXR, pancreatic insufficiency, malabsorption and steatorrhea, nasal polyps

Meconium ileus in newborns.

Infertility in males (absence of vas deferens, absent sperm).

Subfertility in females (amenorrhea, abnormally thick cervical mucus).

Fat-soluble vitamin deficiencies (A, D, E, K).

42
Q

Cystic fibrosis

Treatment?

A

N-acetylcysteine to loosen mucus plugs (cleaves disulfide bonds within mucus glycoproteins),

dornase alfa (DNAse) to clear leukocytic debris.

43
Q

X-linked recessive disorders

A

Bruton agammaglobulinemia, Wiskott-Aldrich syndrome, Fabry disease, G6PD deficiency, Ocular albinism, Lesch-Nyhan syndrome, Duchenne (and Becker) muscular dystrophy, Hunter Syndrome, Hemophilia A and B, Ornithine transcarbamylase deficiency.

Female carriers can be variably affected depending on the percentage inactivation of the X chromosome carrying the mutant vs. normal gene.

Be Wise, Fool’s GOLD Heeds Silly HOpe

44
Q

Duchenne muscular dystrophy

A

X-linked frameshift (deletions, duplications, or nonsense) mutations –>
truncated dystrophin protein Ž–> inhibited muscle regeneration..

Weakness begins in pelvic girdle muscles and progresses superiorly.

Pseudohypertrophy of calf muscles due to fibrofatty replacement of muscle.

Gower maneuver—patients use upper extremity to help them stand up. Waddling gait.

Onset before 5 years of age. Dilated cardiomyopathy is common cause of death.

Duchenne = deleted dystrophin.
Dystrophin gene (DMD) has the largest protein-coding human gene --\> increase  chance of spontaneous mutation. Dystrophin helps anchor muscle fibers, primarily in skeletal and cardiac muscle. It connects the intracellular
cytoskeleton (actin) to the transmembrane proteins a- and b-dystroglycan, which are connected to the extracellular matrix (ECM).

Loss of dystrophin results in myonecrosis. Increased CPK and aldolase are seen;

Western blot and muscle biopsy confirm diagnosis.

45
Q

Becker musclar dystrophy

A

X-linked disorder typically due to non-frameshift insertions in dystrophin gene (partially functional instead of truncated)

Less severe than Duchenne. Onset in adolescence or early adulthood.

Deletions can cause both Duchenne and Becker.

46
Q

Myotonic type 1

A

Autosomal dominant.

CTG trinucleotide repeat expansion in the DMPK gene –>Ž abnormal expression of myotonin protein kinase –>Ž myotonia, muscle wasting, frontal balding, cataracts, testicular atrophy, and arrhythmia.

My Tonia, My Testicles (testicular atrophy), My Toupee (frontal balding), My Ticker (arrhythmia)

47
Q

Fragile X syndrome

A

X-linked defect (breakage) affecting the methylation and expression of the FMR1 gene.

The 2nd most common cause of genetic intellectual disability (after Down syndrome).

Findings: postpubertal macroorchidism (enlarged testes), long face with a large jaw, large everted ears, autism, mitral valve prolapse.

Trinucleotide repeat disorder (CGG)n.
Fragile X = eXtra large testes, jaw, ears.

Diagnosis: culture lymphocytes in folate-deficient or methotrexate –> 4% metaphase have break-point

48
Q

Trinucleotide repeat expansion diseases

A

Huntington disease, myotonic dystrophy, Friedreich ataxia, fragile X syndrome.

Fragile X syndrome = (CGG)n.
Friedreich ataxia = (GAA)n.
Huntington disease = (CAG)n.
Myotonic dystrophy = (CTG)n.

Try (trinucleotide) hunting for my fried eggs (X).

X-Girlfriend’s First Aid Helped Ace My Test.

May show genetic anticipation (disease severity increase and age of onset decrease in successive generations).

49
Q

Down syndrome (trisomy 21)

A

1:700, autosomal trisomies.
Findings: intellectual disability, flat facies, prominent epicanthal folds, single palmar crease, gap between 1st 2 toes, duodenal atresia, Hirschsprung disease, congenital heart disease (ostium primum-type atrial septal defect [ASD]), Brushfield spots.
Associated with early-onset Alzheimer disease (chromosome 21 codes for amyloid precursor protein) and increased risk of ALL, AML.

95% of cases due to meiotic nondisjunction of homologous chromosomes (associated with advanced maternal age; from 1:1500 in women < 20 to 1:25 in women > 45 years old).
4% of cases due to Robertsonian translocation.
1% of cases due to mosaicism (no maternal association; post-fertilization mitotic error).
Drinking age (21).
Most common viable chromosomal disorder and most common cause of genetic intellectual disability.
First-trimester ultrasound commonly shows: increased nuchal translucency and hypoplastic nasal bone; serum PAPP-A is decreased, free β-hCG is increased.
Second-trimester quad screen shows: decreased α-fetoprotein, increased β-hCG, decreased estriol, increased inhibin A.

50
Q

Edwards syndrome (trisomy 18)

A

1:8000, autosomal trisomy

Findings: severe intellectual disability, rockerbottom feet, micrognathia (small jaw), low-set Ears, clenched hands with overlapping fingers, prominent occiput, congenital heart disease.
Death usually occurs within 1 year of birth.
Election age (18).
2nd common trisomy resulting in live birth (most common is Down syndrome).
PAPP-A and free b-hCG are decreased in first trimester.
Quad screen shows: decreased α-fetoprotein, decreased β-hCG, decreased estriol, decreased or normal inhibin A.

51
Q

Patau syndrome (trisomy 13)

A

1:15,000, autosomal trisomy

Findings: severe intellectual disability, rockerbottom feet, microphthalmia, microcephaly, cleft liP/Palate, holoProsencephaly, Polydactyly, congenital heart disease.

Death usually occurs within 1 year of birth.
Puberty (13).
First-trimester pregnancy screen shows: decrease free b-hCG, decrease PAPP-A, and increase nuchal translucency.

52
Q

Chromosomal disorders

A

3: von Hippel-Lindau disease, renal cell carcinoma
4: ADPKD with PKD2 defect, Huntington disease
5: Cri-du-chat syndrome, familial adenomatous polyposis
7: Williams syndrome, cystic fibrosis
9: Friedreich ataxia
11: Wilms tumor
13: Patau syndrome, Wilson disease
15: Prader-Willi syndrome, Angelman syndrome
16: ADPKD with PKD1 defect
17: Neurofibromatosis type 1
18: Edwards syndrome
21: Down syndrome
22: Neurofibromatosis type 2, DiGeorge syndrome (22q11)

X: Fragile X syndrome, X-linked agammaglobulinemia, Klinefelter syndrome (XXY)

53
Q

Robertsonian translocation

A

Nonreciprocal chromosomal translocation that commonly involves chromosome pairs 13, 14, 15, 21, and 22. One of the most common types of translocation. Occurs when the long arms of 2 acrocentric chromosomes (chromosomes with centromeres near their ends) fuse at the centromere and the 2 short arms are lost. Balanced translocations normally do not cause any abnormal phenotype. Unbalanced translocations can result in miscarriage, stillbirth, and chromosomal
imbalance (e.g., Down syndrome, Patau syndrome).

54
Q

Cri-du-chat syndrome

A

Congenital microdeletion of short arm of chromosome 5 (46,XX or XY, 5p-).
Findings: microcephaly, moderate to severe intellectual disability, high-pitched crying/mewing, epicanthal folds, cardiac abnormalities (VSD).

Cri du chat = cry of the cat.

55
Q

Williams syndrome

A

Congenital microdeletion of long arm of chromosome 7 (deleted region includes elastin gene).
Findings: distinctive “elfin” facies, intellectual disability, hypercalcemia (increased sensitivity to vitamin D), well-developed verbal skills, extreme friendliness with strangers, cardiovascular problems.

56
Q

22q11 deletion syndromes

A

Microdeletion at chromosome 22q11 –> variable presentation, including Cleft palate, Abnormal facies, Thymic aplasia –>Ž T-cell deficiency, Cardiac defects, and Hypocalcemia 2° to parathyroid aplasia
DiGeorge syndrome—thymic, parathyroid, and cardiac defects.
Velocardiofacial syndrome—palate, facial, and cardiac defects.

CATCH-22.

Due to aberrant development of 3rd and 4th branchial pouches.

57
Q

Fat soluble vitamins

A

A, D, E, K. Absorption dependent on gut and pancreas. Toxicity more common than for water-soluble vitamins because fat-soluble vitamins accumulate in fat.
Malabsorption syndromes with steatorrhea, such as cystic fibrosis and sprue, or mineral oil intake can cause fat-soluble vitamin deficiencies

58
Q

Water soluble vitamins

A

B1 (thiamine: TPP)
B2 (riboflavin: FAD, FMN)
B3 (niacin: NAD+)
B5 (pantothenic acid: CoA)
B6 (pyridoxine: PLP)
B7 (biotin)
B9 (folate)
B12 (cobalamin)
C (ascorbic acid)
All wash out easily from body except B12 and folate (stored in liver).
B-complex deficiencies often result in dermatitis, glossitis, and diarrhea.

59
Q

Pyruvate carboxylase deficiency

A

Gluconeogenesis (MT): Pyruvate + CO2 + ATP –> Oxaloacetate + ADP + Pi

Activated by Acetyl-CoA; requires biotin

Citric acid cycle downregulated –> downregulation of energy-producing capacity of the cell

High pyruvate, high alanine (no conversion to pyruvate); high lactate, high ketoacids

Hepatocytes: essential in gluconeogenesis from alanine; converts lactate to glucose

Allelic heterogeneity: NA moderate = mental retardation, lactic acidosis; Great Britain/France life-threatening = die before 3 months

60
Q

Neonatal jaundice/hyperbilirubinemia

A

Common (60% term, 80% preterm); accumulation ofunconjugated bilirubin secondary to hemolysis of fetal RBCs (replaced by HgA) and/or low activity of UDP-glucuronyl transferase

Severely elevated bilirubin –> kernicterus (deposition in basal ganglia) –> permanent neurologic complications in infants

Boys, East Asian

Treatment: phototherapy decomposes to water-soluble into urine –> also destroys riboflavin –> supplement too

61
Q

Tyrosinase deficiency

A

Albinism

Congenital deficiency of tyrosinase or tyrosine transporter

Reduced pigmentation of skin, eyes, hair –> increase risk of skin cancer

62
Q

Trisomy 16

A

Most common trisomy among abortuses and never encountered in live births

Fertilization of an egg by two sperm

63
Q
A

First Aid for the USMLE Step 1 (1 decks)

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