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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. What Abs block the binding between aminoacyl tRNA and rRNA by binding the 30S ribosome?
Tyrosine; phenylaline
Carbamoyl phosphate to orotic acid
D2 (ergocalciferol) - D3 (cholecalciferol) - D3 (cholecalciferol)
Aminoglycosides and tetracyclines

2. In the fasting state - What ensures that the pyruvate doesnt get used for TCA cycle?
The products of FA oxidation (Acetyl CoA and NADH) phosphorylate pyruvate deyhdrogenase Which inactivates it
Blood group types
Mucus secreting goblet cells - ab secreting plasma cells - nt secreting neurons
homocysteine transferase

3. Where are receptors for Vit D? (think How does it increase serum Ca and Phosphate)
Acetyl coA; gluconeogenesis
Intestine - renal - bone
AR
Malabsorption problems (CF and sprue) and mineral oil intake

4. What is the order of cell cycle phases? What happens in each phase?
Mitosis then Interphase (G1 - S - G2); in G1 is presynthesis phase - S is synthesis phase (DNA rep. etc) - G2 is premitotic (making of MT spindle etc)
Converts it to citrulline
An AP (apyrimidinic) endonuclease to cut the DNA - exonuclease then removes and then filled and sealed by enzymes
Multiple bone fractures - blue sclerae - hearing loss (abdnormal middle ear bones!) - dental imperfections - retarded wound healing

5. What are the mucopolysaccharidoses? Which one is XR? AR? What accumulates in them?
can be d/t lyonization
Vomiting - rice water stools - garlic breath
CPSII
Hurlers Syndromes;Iduronate sulfatase deficiency; alpha L iduronidase def; heparan sulfate - dermatan sulfate

6. What is used in a marathon?
Glycogen and FFA oxidation - glucose conserved for final sprinting!
Nucleus
Downs
Lens and neural tissue; converts galactose to osmotically active galactitol

7. Fructose 2 -6 bisphophate is involved in...
Hepatic glycogenolysis (mostly) - also hepatic gluconeogenesis - adipose release of FFA (minor)
Immediately after replication - DNA is unmethylated and so it is a signal to be double checked; HNPCC; heterochromatin
Mitochondria - archaebacteria - mycoplasma - and some yeasts
allosteric regulation

8. Vit A is ________ - causes cardiac problems and cleft palate in newborns
Phenylalanine hydroxylase; tetrahydrobiopterin factor
Severe fasting hypoglycemia - very high levels glycogen in liver - high blood lactate - hepatomegaly
teratogenic
Robertsonian translocations

9. What type of exonuclease activity does DNA polymerase I have? in What organisms is it found? What is it used for?

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10. What are the general symptoms of 22q11 chromosomal deletion? why is it that these occur?
Pernicious anemia; older - mentally slow european woman who is anemic and icteric with a smooth shiny tongue and has broad based shuffling gait
CNS defects - mental retardation - death - feeding difficulties - vomiting - hypoglycemia; thiamine
The active form of Vit D (1 -25 (OH)2 D3)
Cleft palate - Abnormal faces - thymic aplasia (t cell def) - Cardiac defects - hypocalcemia (secondary to hypoparathyroid); 3rd and 4th branchial pouch aberrant development

11. What is the difference between methotrexate and trimethroprim?
COP II
Isocitrate dehydrogenase
Vitamin B1 (thiamine) - Vitamin B2 (riboflacin) - Vitamin B3 (niacin) - Vitamin B5 (pantothenate) - Vitamin B6 (pyridoxine) - Vitamin B7 (Biotin) - Vitamin B12 (cobalamin) - folate - Vit. C
Trimethroprim inhibits bacterial dihyrofolate reductase

12. What are the three main cytoskeletal elements?
Actin/myosin - MT - intermediate filaments
CPSII
Carnitine acyltransferase I
SER and mitochondria; NADPH

13. What is the degradation product of dopamine?
Pyruvate carboxylase (pyruvate to OXA) (mit) - PEP carboxykinase (OXA to PEP) (cyt) - Fructose 1 -6 bisphophatase (fructose 1 -6 bisphosphate to fructose 6 P) (cyt) and glucose 6 phosphatase (in ER); glucose 6 phosphatase
HVA
Mebendazole and thiabendazole
AR

14. High blood levels of fructose - galactose - glucose can result in conversion to osmotically active alcohol forms by...
fructose
Uses HMG coA reductase
B12 and folate; B12; washed out
aldose reductase

15. What is the RNA called before processing? and after?
HnRNA; mRNA
When the amino acid is covalently binded to its corresponding tRNA; Aminoacyl tRNA synthetase; ATP; translation
Mitosis then Interphase (G1 - S - G2); in G1 is presynthesis phase - S is synthesis phase (DNA rep. etc) - G2 is premitotic (making of MT spindle etc)
Neurologic defects - myopathy - lactic acidosis; giving ONLY ketogenic nutrient high fat content and aa (lysine and leucine)

16. How does mismatch repair work? in What cancer is it mutated? What type of DNA is methylated?
Dopamine
Isocitrate dehydrogenase
Immediately after replication - DNA is unmethylated and so it is a signal to be double checked; HNPCC; heterochromatin
Double strand breaks; nonhomologous end joining

17. What is a nissl body?
RER in nucleus (synthesize NTs)
Biotin; rate limiting step in fA synthesis - acetyl coA into malonyl coA
Tyrosinase deficiency; defective tyrosine transporters; lack of migration of neural crest cells; skin cancer
Schwann cells - lens - retina - kidneys because they only have aldose reductase and no sorbital dehydrogenase

18. What are the glycogen storage disease? What they result in?
Abnormal glycogen metabolism results in accumulation of glycogen in cells
pleiotropic
So that excess glucose can be stored (phosphorylated glucose cant leave) in the liver and saved as a buffer
Dark connective tissue - brown pigmented sclera - debilitating athralgias; homogentistic acid builds up and can be toxic to cartilage

19. How is muscular dystrophies diagnosed?
CNS defects - mental retardation - death - feeding difficulties - vomiting - hypoglycemia; thiamine
Increased CPK and muscle biopsy
IMP
dimers

20. What converts dopa to dopamine? using What cofactor?
Riboflavin; dairy and meat
Hemolytic anemia due to heinz bodies Which are oxidized hemoglobin precipitating within the RBCs
Induction in chronic alcoholism (increases tolerance); inhibition in acute alcoholism
Dopa decarboxylase; B6

21. What syndrome is characterized by hyperextensible skin and joints - and bleeding? What is the most common reason? What are other clinical symptoms?
glutton
1) if it is stable/labile or permanent 2) intact BM and 3) intact ECM
A fibrous protein (ie produced by fibroblasts); it has lysine and proline but nonglycosylated; arteries - lungs - elastic ligaments - vocal cords - ligamenta flava (connect vertebrae); broken down by elastase Which is inhibited by alpha 1 antitrypsin
Ehler Danlos; type III collagen def; joint dislocation - berry aneurysm - ecchymoses - organ rupture

22. What aa is most accumulated in hartnup disease? What vitamin becomes deficient?
Tryptophan; niacin
Nacetyl cysteine; breaks disulfide bridges of mucus plugs
Coenzyme A and lipoamides
Phenylalanine build up

23. What is the rate determining enzyme in HMP shunt?
Peripheral neuropathy - developmental delay - optic atrophy - globoid cells
Glucose 6 phosphate dehydrogenase
Mitosis then Interphase (G1 - S - G2); in G1 is presynthesis phase - S is synthesis phase (DNA rep. etc) - G2 is premitotic (making of MT spindle etc)
B hydroxybutyrate - acetone - acetoacetate; beta hydroxybutyrate; beta hydroxybutyrate

24. What regulates Fructose 1 -6 bisphosphatase? What reaction is it found in?
Constitutive - random insertion of gene into mouse genome; conditional - targeted through homologous recombination
Downs
Citrate is positive on it - AMP and fructose 2 -6 bisphosphate is negative on it; gluconeogenesis
Fruits and vegetables

25. What luminal secretions are affected in CF? What is wrong with the secretions?
To increase serum Ca and phosphate for bone deposition
Respiratory - seminal fluid - pancreatic - gallbladder; very thick without NaCl to pull water
Ret
Aldolase B; Fructose 1 phosphate accumulates Which uses up Phosphate and the result is inhibition of gluconeogenesis without phosphate; hypoglycemia - jaundice - cirrhosis - vomitting

26. What is this reaction dependent on?
Arylsulfatase A; cerebroside sulfate
Oxygen!
Induction in chronic alcoholism (increases tolerance); inhibition in acute alcoholism
S adenosyl homocysteine; homocysteine transferase; B12 and THF; M- THF

27. why does galactose def present in newborns>
Carnitine shuttle
Presence of normal and mutated mtDNA (like mosaicism except in mit)
Tryptophan; niacin
Lactose in mothers milk contains galactose

28. What is the inheritance of CF? What is the pathogenesis of the disease?
Tyrosine
Pyridoxine; converted to pyridoxal phsophate used in transamination reaction - decarboxylations - heme synthesis - niacin synthesis from trp - glycogen phosphorylase - cystathionine synthesis - NT synthesis
1) if it is stable/labile or permanent 2) intact BM and 3) intact ECM
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

29. What type of enyzmes are used in base excision repair?
MT!
Glucokinase
Alcoholism - RA - inflammatory disease - chronic diarrhea
Glycosylases (only remove the base - cut the bond between ribose and base)

30. Which antigout drugs act on MT?
Night blindness - dry skin
GTP: gluconeogenesis - OXA to phosphoenolpyruvate
Cancers (rapidly synthesizing); MTX (blocks dihyrdofolate reductase); phenytoin (prevent conjugase from breaking down polyglutamate form to monoglutamate form; reabsorption in jejunum is blocked by alcohol and OCPs; also blocked by sulfonamides
Colchine

31. What is the exception to the nonoverlapping/commaless aspect of the genetic code?
Some viruses
Very Poor Carb Metabolism (Von Gierke - Type 1 - Pompes - Type II - Cori's - Type III - McArdles disease - Type IV)
CPSII
Tryptophan

32. What is main lab finding in Lesch Nyhan syndrome? due to What enzyme deficiency? What reaction is deficient?
Uric acidemia; HGPRT; purine salvage
Catalase positive because they reduce their H2O2 and WBCs cant use that to make bleach; because the way they protect is by building a wall around the organisms with granulomas
Phenylketones that build up in phenylketonuria
No; increased mutation rate!

33. Other than the H2O2 produced from the resp burst in infections - What other things does G6PD reproduce NADPH for glutathione reduction for? What cells are particularly vulnerable to these oxidizing agents?
Sulfonamides - primaquine - dapsone - antiTB drugs - fava beans - LDL; RBCs
CPSII
Glutathione peroxidase (catalase) reduces H202 to H20 with glutathione; oxidized glutathione gets reduced by glutathione reductase with NADPH; NAP+ is regenerated to NADPH with Glucose 6 phosphate dehydrogenase; because H2O2 from the resp burts in th
6MP blocks de novo purine synthesis

34. What 3 genetic disorders can cause Marfanoid habitus?
Metanephrine
Marfans - homocystinuria - MEN 2B
Coenzyme A and lipoamides
COMT and MAO

35. Which metabolic reactions require both the mit and cytosol?
Heme synthesis - urea cycle - gluconeogenesis
Hepatic glycogenolysis (mostly) - also hepatic gluconeogenesis - adipose release of FFA (minor)
Phenylethanolamine N- methyl transferase; SAM
The active form of Vit D (1 -25 (OH)2 D3)

36. How does a kwashiokor patient present?
G6PD; provides malarial resistance; X linked recessive
Pellagra
Inhibits lipoic acid; lipoic acid is a cofactor for pyruvate dehydrogenase
Malnutrition - edema (hypoalbuminemia) - anemia liver (fatty change) - diarrhea d/t no brush border enzymes (no protein!)

37. What dose a deficiency in Vit B12 cause? What causes it?
32 or 30; malate aspartate shuttle or glycerol 3 phosphate shuttle; heart/liver and muscle
Fructose 1 -6 bisphosphatase
Decreased would healing; microcytic anemia (d/t decreased iron [not absorbable]; macrocytic anemia (d/t Fh4 not reduced)
Macrocytic megaloblastic anemia - hypersegmented PMNs - subacute combined degeneration myelopathy Which results in degeneration of the dorsal and lateral columns (loss of position and vibration sense - spastic paresis and ataxia) due to abnormal myel

38. What are two examples of X linked Autosomal Dominant disease?
Ouabain
Hepatic glycogenolysis (mostly) - also hepatic gluconeogenesis - adipose release of FFA (minor)
hexokinase (glucokinase only found in liver and beta cells of pancreas)
Hypophosphatemia rickets and Alports

39. What type of vitamin is Vit D? Where do we get it? in What form?
The lack of lysosomal proteins (are secreted - default from RER!) results in macromolecules accumulating in lysosomes - inclusions cause pathology
Its the ribose; its made from PRPP synthetase using ribose 5 P from the HMP shunt
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)
B hydroxybutyrate - acetone - acetoacetate; beta hydroxybutyrate; beta hydroxybutyrate

40. What amino acids are required in purine synthesis? Which of these does pyrimidine synthesis require also?
AD
Helps make Vit K and antioxidant
Glutamine - aspartate - glycine; aspartate
tubulin dimer

41. Other than thymidylate synthase - What else is required to convert dUMP to dTMP?
N5N10 methylene THF
Cell signaling defect in FGF receptor resulting in dwarfism and short limbs with normal head and trunk; AD
Chloramphenicol
1 gene has greater than 1 phenotypic effect; locus heterogeneity (many different mutations can cause the same phenotype); PKU; albinism - deafness - Marfanoid habitus (Marfans - MEN 2B - homocystinuria)

42. What does methotrexate do? What step is this enzyme involved in?
adrenal medulla
Cobalamin; cofactor for homocysteine methyltransferase (transfers SAM to THF to give to dUMP) and for methylmalonyl coA mutaseto make succinyl coA
It inhibits conversion of DHF to THF by blocking dihyrdofolate reductase
CNS defects - mental retardation - death - feeding difficulties - vomiting - hypoglycemia; thiamine

43. Where are cilia found? What symptoms are associated with kartageners syndrome?
Wrinkles - acne and psoriasis topically - promyelocytic anemia - measles
So that excess glucose can be stored (phosphorylated glucose cant leave) in the liver and saved as a buffer
In the reproductive tract and in the resp tract; infertility - bronchiectasis and recurrent sinusitis - associated with situs inversus
jaundice

44. After the PCR reaction - How do you actually visualize the DNA?
only yield acetyl coA
Arylsulfatase A; cerebroside sulfate
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
Do agarose gel eletrophoresis to separate the different sizes of the PCR products (sizes id the the fragment)

45. What are heat shock proteins?
Assist in spontaneous refolding of proteins
Water soluble vitamin stored in liver; converted to THF by dihydrofolate reductase and is a coenzyme in 1 methyl transfers - impt for DNA synth
Von gierkes
When there is glucagon; protein kinase A activation and thus phosphorylation of the complex resulting in the activation of fructose 2 -6 BPase and no PFK2

46. What is the main source of FA? Where else can they be released from?
NADPH oxidase; CGD
Increased G3P for TG synthesis - increased acetyl CoA (Fatty acid synthesis for TG synthesis) - and decreased oxidation of FA in the mitochondria due to alcohols toxicity on the mit
Decreased phenylalanine - increase tyrosine
Synthesis in liver and then hydrolysis of chylomicrons and VLDLs by capillary lipoprotein lipase in peripheral tissue; adipose tissue by lipases

47. What are the symptoms of cri du chat syndrome? What is the pathogenesis?
Microcephaly - mental retardation - high pitched mewing/crying - epicanthal folds - cardiac abnormalities; microdeletion on short arm of chrom 5
DNA dependent RNA polymerase
Lime and CaCO3 to displace the niacin
Lysine and arginine; basic

48. What is the first enzyme and step in ethanol metabolism? second step? What is a cofactor in both of these steps? What vitamin does this come from?
Isocitrate dehydrogenase
Lipoprotein lipases take FA from VLDL and chylomicrons Which are then combined with glycerol 3 P (a carb intermediate) to make triacylglycerol
Developmental delay - gargoylism - airway obstruction (d/t short neck) - corneal clouding - CAD - hepatosplenomegaly
Alcohol dehyrogenase (alcohol to acetaldehyde); acetaldehyde dehydrogenase (acetaldehyde to acetate); NAD+; B3 (niacin)

49. What inheritance pattern is Type IIA hypercholesterolemia? do heterozygotes present with disease? do homozygotes present?
Lys and arg
AD: yes! most common presentation; very rare (700 mg/dl cholesterol level!)
Mit encephalopathy - lactic acidosis - stroke like episodes
DNA dependent RNA polymerase

50. How do digoxin and digitoxin work? What class of drugs are they?
Increased blood levels of ammonia - decreased BUN
low Vmax
kidney
Inbit Na+K+ ATPase; Cardiac glycosides