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USMLE Step 1 Biochemistry

Subjects : health-sciences, usmle-step-1

Instructions:

Answer 50 questions in 15 minutes.
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Match each statement with the correct term.
Don't refresh. All questions and answers are randomly picked and ordered every time you load a test.

This is a study tool. The 3 wrong answers for each question are randomly chosen from answers to other questions. So, you might find at times the answers obvious, but you will see it re-enforces your understanding as you take the test each time.

1. Which amino acid is the most basic?
Arg
High glycogen in muscle - but cannot break it down - leading to painful muscle cramps - myoglobinuria with strenuous exercise - no increase in lactic acid after exercise; skeletal muscle glycogen phophorylase
Tyrosine
Cafe au lait spots - optic pathway gliomas - lisch nodules - scoliosis - neural tumors

2. Is hexokinase or glucokinase activated by insulin?
Glucokinase
Mannose
tumor suppressor genes (p53 - Rb - BAX)
Its When two acrocentric chromosomes combine their long arms (and short arms disappear); When the translocation is functional

3. No offspring of affected males with a mit disease can...
show disease
Cleft palate - Abnormal faces - thymic aplasia (t cell def) - Cardiac defects - hypocalcemia (secondary to hypoparathyroid); 3rd and 4th branchial pouch aberrant development
Galactose 1 phosphate uridyltransferase (GALT) results in an accumulation of toxic substances; failure to thrive - jaundice - hepatomegaly - infantile cataracts - mental retardation; exclude galactose and lactose from diet
Glutamate and aspartate; CO2 provides an ketone

4. What results in marasmus? What kind of muscle wasting? What kind of muscle wasting do you see in kwashiokor?
A folic acid analogue
Loss of subcutaneous fat - tissue and muscle wasting (somatic muscle); visceral - not somatic
Def in Vit C; def in Vit D
Dry is just the symmetrical neuropathy with both motor and sensory symptoms; wet includes both the neuropathy and cardiac involvement of high output failure with peripheral edema - tachycardia - cardiomegaly

5. Like mRNA has a poly A tail on the 3' end - What does tRNA have on its 3' end? What happens here?
By cyclin CDK complexes
Riboflavin makes up FAD and FMN both used in redox reactions; vegans
Vit K; neonates have sterile intestine and can not synthesize vit K
CCA and chemically modified bases Where amino acid is covalently bonded

6. What syndrome is characterized by hyperextensible skin and joints - and bleeding? What is the most common reason? What are other clinical symptoms?
Orotic aciduria megaloblastic anemia can not be corrected with folic acid or vit B12
Cleft palate - Abnormal faces - thymic aplasia (t cell def) - Cardiac defects - hypocalcemia (secondary to hypoparathyroid); 3rd and 4th branchial pouch aberrant development
Ehler Danlos; type III collagen def; joint dislocation - berry aneurysm - ecchymoses - organ rupture
fastest sugar

7. What is used in a marathon?
Arginine
Glycogen and FFA oxidation - glucose conserved for final sprinting!
By epoxide reductase in the liver; coumadin; intestinal flora and leafy vegetables
Beta glucocerebrosidase; glucocerebroside

8. What is phenylethanolamine N- methyl transferase located?
Milder form of type 1 with normal blood lactate levels; debranching enzyme (alpha -1 -6 glucosidase)
Adrenal medulla
HMG CoA synthetase; HMG CoA reductase
Galactose gets converted to galactose 1 Phosphate by galactokinase Which then gets turned into glucose 1 phosphate by Galactose 1 Uridyl Transferase (GALT) Which also produces UDP Gal Which is used for lactose production in the breast; the fate of gl

9. What are the first three enzymes in the first three steps in respiratory burst? Where do they occur? What do they result in? What does it do?
Vomiting - rice water stools - garlic breath
NADPH oxidase - superoxide dismutase - myeloperoxidase; in the phagolysosome; HOCl (bleach); kills!
Dihydrofolate reductase; vitamin B12; homocysteine
Malabsorption problems (CF and sprue) and mineral oil intake

10. From What aa does glutathione come from?
Formation of a proton gradient that is then used to create ATP through ATP synthase
Fomepizole; antidote for methanol or ethylene glycol poisoning
To increase serum Ca and phosphate for bone deposition
Glutamate

11. What is the product of odd chain fatty acid synthesis? How does it return to TCA cycle? What cofactor is required?
Lack of lactase (brush border enzyme) so can not break down lactose (a dissaccharide of galactose and glucose); African Americans and Asians
3 NADH - 1 FADH2 - 2 CO2 - 1 GTP = 12 ATP/acetyl CoA
Sulfonamides - primaquine - dapsone - antiTB drugs - fava beans - LDL; RBCs
Proprionyl coA; through methylmalonyl coA; B12

12. What is the difference between a kinase and a phophorylase?
DOPA
tubulin dimer
Both add phosphate; phosphorylase uses no ATP and inorganic phosphate
Limit protein in diet; give benzoate or phenylbutyrate Which bind amino acids and lead to excretion - lactulose (hydrogen ion binds to ammonia to make excretable ammonium) and neomycin (to kill bacteria that release ammonia from aa)

13. AR disease are usually...
Citrate shuttle in the inner mitochondrial membrane transports acetyl coA into the cytoplasm; liver; liver and adipose tissue
Steroid synthesis and detoxification
enzymatic
Stored ATP - creatine phosphate - anaerobic glycolysis; as distances increase - ATP is obtained from additional resources; above plus ox phos

14. What cofactor is required for lysyl oxidase cross linking? What reaction is required beforehand for crosslinking to occur?
Alpha galactosidase A; ceramide trihexoside
Histidine; B6
Cu; hydroxylation
Thinned cornea from lack of connective tissue so see underlying choroidal lens

15. What is the structure of Microtubules?
Cylindrical made up of a helical array of polymerized alpha and beta tubulin dimers - each dimer has 2 GTP bound
Deficiencies in one of the many lysosomal enzymes results in accumulation of abnormal metabolic products
30S ribosomal RNA (small one)
Tyrosine

16. What is the result of pyruvate dehydrogenase deficiency? What causes it?
Carbomyl phoshpate synthetase I
COP I
Sickle cell disease - conversion of hydroophillic glutamic acid to hyrdophobic valine
Back up of pyruvate and alanine and thus lactic acidosis occurs; can be d/t alcoholism (vit B1 def)

17. What type of vitamin is Vit D? Where do we get it? in What form?
Fat soluble; either endogenous production or exogenous; D2 ergocalciferol is ingested from plants and D3 cholecalciferol is formed in sun exposed skin (and can be consumed in fortified milk)
Histidine; B6
In ability to reabsorb phosphate from renal tubular cells results in osteomalacia due to defective bone mineralization; no
dimers

18. Hexokinase has a HIGH affinity (low Km) and...
All clathrin
low Vmax
Phosphoenolpyruvate; pyruvate kinase; fructose 1 - 6 BP; ATP - alanine
ATP and biotin

19. What does severe deficiency of B3 lead to? What are the causes?
(cofactor for 100+ enzymes!) needed for zinc fingers transcription motif - collagen synthesis - alcohol metabolism
AR; trinucleotide deletion results in loss of a Phe aa on CFTR gene on chrom 7- the result is a mutated CFTR channel that gets degraded in the ER before reaching the surface; the purpose of CFTR is to increase Cl secretion in luminal secretions and r
Severe fasting hypoglycemia - very high levels glycogen in liver - high blood lactate - hepatomegaly
Pellagra (dermatitis - diarrhea - dementia); INH use - B6 deficiency - Hartnup disease - malignant carcinoid syndrome - and a corn based diet (lacks trp and niacin in corn cant be absorbed)

20. How do you treat niacin deficiency do to corn based diet?
Decrease intake of fructose and sucrose (fructose and glucose)
Lime and CaCO3 to displace the niacin
SnRNPs; RNA polymerase III
fructose

21. After citrulline is produced and in the cytoplasm What happens next in the urea cycle?
low Vmax
When water is pulled in; polyols (sugar alcohols); cataracts - retinopathy - peripheral neuropathy all seen in chronic hyperglycemia in diabetes
TFs
Citrulline is combined with aspartate to make arginosuccinate Which is then split into fumarate (which leaves to enter TCA) and arginine. Arginine is then converted to Ornithine (via arginase) giving off Urea Which then goes to the kidney and ornithi

22. What is carnitine deficiency and What does it result in?
Aromatic; musty body odor
Night blindness - dry skin
Inability to transport LCFA into mitochondria results in their build up and causes weakness - hypotonia And HYPOKETOTIC HYPOGLYCEMIA
Convulsions - hyperirritiability - peripheral neuropathy - sideroblastic anemias; alcoholics and INH and fortified goats milk consumption

23. What is the other name of B3? What is its function? Synthesis of B3 requires What other vitamin? What else?
SER and mitochondria; NADPH
Type I collagen; AD
Niacin/Nicotinic acid; NAD+ and NADH used in redox reactions - and catabolic and anabolic reactions; B6; tryptophan
AR

24. What is dysgeusia?
Fructose gets phosphorylated into fructose 1- P by fructokinase and then fructose 1P gets metabolized into DHAP or glyceraldehyde by aldolase B
Fat malabsorption (CF) and abetalipoproteinemia (damage takes up all antioxidants)
Inability to taste
When the 2nd hit of the allele is What causes the disease; sporadic retinoblastoma

25. What is the presentation of G6PD deficiency?
A methyl
Hemolytic anemia; jaundice in newborn if mom gets too much vit K
paternal
Hemolytic anemia with a blood smear with heinz bodies and bite cells (phagocytic removal of heinz bodies)

26. What is the degradation product of Norepinephrine?
Some viruses
Phosphofructokinase 1
I; nucleolus
VMA

27. Where is vit C found?
Fruits and vegetables
Ceramide trihexoside
All clathrin
Increased TG synthesis from increased Glycerol 3 P and abetalipoproteinemia

28. What are the symptoms of B2 deficiency?
Protein Kinase A inhibits pyruvate kinase also ATP and alanine have negative effects on pyruvate kinase
Cheilosis and corneal vascularization
GALT
Cystathionine (which then goes to cysteine) via cystathionine reductase; B6

29. deamination of What pyrimidine makes what?
Cheilosis and corneal vascularization
Antineoplastic agents; alkylate DNA!; cyclophosphamide and nitrosurea (aka lomustrine - a nitrosource)
Cytosine to uracil
Lys and arg

30. At What point is the amino acid correspondence to the mRNA code finalized?
To increase serum Ca and Phosphate for ultimate bone deposition by: increase intestinal absorption of calcium and phosphate (also renal); and increase resorption of bone by stimulating macrophages (osteoclasts)
Vit C deficiency; glossitis - anemia - weakened vessels - hemorrahges - hemarthroses - swollen gums - bruises -
In the charging reaction! for this reason aminoacyl tRNA synthetase double checks
Homocysteine; homocysteine methyltransferase with B12 and tetrahydrofolate

31. What is the source of Biotin?
Vit C deficiency; glossitis - anemia - weakened vessels - hemorrahges - hemarthroses - swollen gums - bruises -
As alanine; amino acids donate their amino group to alpha ketoglutarate making it glutamate and the aa then becomes an alpha ketoacid (joins TCA cycle); glutamate then converts pyruvate to alanine by donating its amino group to pyruvate (with ALT); a
Bacterial synthesis in the intestine
Cystic Fibrosis (CFTR gene)

32. What causes an excess of B3? What are the symptoms?
Niacin treatment for hyperlipidemia; facial flushing
Because that is Where the products will be consumed; acetyl coA; TCA cycle of ketone body production
NF1
Vit C

33. With What tool are Prader Willi and Angelman diagnosed?
FISH
Oxygen!
NADPH oxidase - superoxide dismutase - myeloperoxidase; in the phagolysosome; HOCl (bleach); kills!
fatty acid degradation

34. What is the result of arginase deficiency?
Southern - DNA - Northern - RNA - Western - Protein; blotting procedures are used to detect a specific nucleic acid or aa sequence; the sample is electrophoresed and then transferred to a filter Which is then soaked in a denaturant and then a radiola
False; GTP hydrolysis
succinyl coA (and then to OXA)
Chorea and spastic paresis

35. What is the main clinical presentation of hartnup disease?
Release alk phos
Glycine; proline and lysine
Pellagra
Alanine aminotransferase (ALT) and aspartate aminotransferase (AST); alanine to pyruvate and aspartate to oxaxloacetate

36. What is the treatment for phenylketonuria?
Smoking (FR takes up all the antioxidants); diets lacking fruit
Hemolytic anemia (fragility of RBCs) - muscle weakness and posterior column and spinocerebellar tract demyelination (neuron damage) resulting in decreased proprioception and vibration sensation - and peripheral neuropathy - and retinal degeneration
Decreased phenylalanine - increase tyrosine
Trinucleotide expansion diseases

37. What is needed after the glycosylase in base excision repair?
MT!
Mitochondria in liver; HmG CoA synthetase; amino acids and beta oxidation of FA
An AP (apyrimidinic) endonuclease to cut the DNA - exonuclease then removes and then filled and sealed by enzymes
Vimentin - desmin - cytokeratin - glial fibrillary acidic protein (GFAP) - neurofilaments; to differentiate between different cells

38. What is the rate determining enzyme in gluconeogenesis?
Either salvage by HGPRT and PRPP or converstion to xanthine and then to uric acid by xanthine oxidase
Its the ribose; its made from PRPP synthetase using ribose 5 P from the HMP shunt
A topoisomerase II inhibitor; because cancer cells use II more so than healthy cells
Fructose 1 -6 bisphosphatase

39. Which glycogen storage disease results in hepatomegaly?
CPSII
Tyrosine hydroxylase; tetrahydrobiopterin factor; NADP+
Von gierkes
Kartaganer's aka ciliar dyskinesia; lack of dynein arm ATPase

40. What dictates whether a cell will regenerate or repair after inflammation?
Pataus
Incoming Aminoacyl tRNA; site Where peptidyltransferase bond is made between growing peptide aa's; the holds the empty tRNA as it exits; the initiator mRNA (met or f - met) starts off in E site rather than A site
Acetyl CoA carboxylase
1) if it is stable/labile or permanent 2) intact BM and 3) intact ECM

41. What is the mode of inheritance of Huntingtons?
Yes! congenital simply means 'born with'- hereditary is familial
AR; trinucleotide deletion results in loss of a Phe aa on CFTR gene on chrom 7- the result is a mutated CFTR channel that gets degraded in the ER before reaching the surface; the purpose of CFTR is to increase Cl secretion in luminal secretions and r
Vit C
AD (trinucleotide expansion is not a mode of inheritance)

42. Which histone ties DNA- histone nucleosome beads into more condensed structures?
H1
Increased TG synthesis from increased Glycerol 3 P and abetalipoproteinemia
Chloramphenicol
Decrease intake of fructose and sucrose (fructose and glucose)

43. The __________ adds O linked oligosaccharides to serine and threonine residues on proteins
golgi apparatus
Myotonic Dystrophy (CTG) - Huntington (CAG) Friedricks Ataxia (GAA) - Fragile (CGG)
Nucleus
Assist in spontaneous refolding of proteins

44. What vesicular trafficking protein takes proteins from the Golgi to lysosomes? in between the golgi? from the plasma membrane to endosomes (receptor mediated endocytosis)?
Excreted from the kidney in the form of urea in hepatocytes
All clathrin
Heme synthesis - urea cycle - gluconeogenesis
BM; laminins; most flexible; different aa composition

45. What is the other name for VIt b12? What is its function?
As BUN (blood urea nitrogen)
Cobalamin; cofactor for homocysteine methyltransferase (transfers SAM to THF to give to dUMP) and for methylmalonyl coA mutaseto make succinyl coA
Phenylalanine
Synthesis in liver and then hydrolysis of chylomicrons and VLDLs by capillary lipoprotein lipase in peripheral tissue; adipose tissue by lipases

46. name three glucogenic AA and how they enter gluconeogenesis. through What cycle do they enter gluconeogenesis?
Carbomyl phosphate synthetase I
Oral uridine administration; provides nucleosides and provides feedback inhibition
Alanine (through pyruvate) - aspartate (through OXA) - glutamate (alpha ketoglutarase); TCA
Prolonged Ab use

47. What is the inheritance of CF? What is the pathogenesis of the disease?
An aminotransferase enzyme transfers an amino group from an amino acid on to alpha ketoglutarate making it a glutamate (to join the urea cycle or excreted in the kidney) and the amino acid that lost the amino group is then turned into a glycolytic in
AR; trinucleotide deletion results in loss of a Phe aa on CFTR gene on chrom 7- the result is a mutated CFTR channel that gets degraded in the ER before reaching the surface; the purpose of CFTR is to increase Cl secretion in luminal secretions and r
Acetyl coA; gluconeogenesis
Abetalipoproteinemia and increased G3P production from all the carbs!

48. What does 6 mercaptopurine do?
Hepatic glycogenolysis (mostly) - also hepatic gluconeogenesis - adipose release of FFA (minor)
Assist in spontaneous refolding of proteins
6MP blocks de novo purine synthesis
Adrenal cells (steroid synthesis) - hepatocytes (p450 detox)

49. What is the importance of folic acid?
Prevents neural tube defects in 1st trimester
severe
Mcardles disease
Cobalamin; cofactor for homocysteine methyltransferase (transfers SAM to THF to give to dUMP) and for methylmalonyl coA mutaseto make succinyl coA

50. What causes scurvy? What are the symptoms of scurvy?
Osteomalacia is vit D def seen in adults and causes bowed legs and results in increased fractures
Vit C deficiency; glossitis - anemia - weakened vessels - hemorrahges - hemarthroses - swollen gums - bruises -
Dopa decarboxylase; B6
The products of FA oxidation (Acetyl CoA and NADH) phosphorylate pyruvate deyhdrogenase Which inactivates it