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USMLE Step 1 Biochemistry

Subjects : health-sciences, usmle-step-1

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Answer 50 questions in 15 minutes.
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1. With hypervitaminosis D you will see increased ______ - it causes bone turnover - and stimulates osteoblasts also
Constitutive - random insertion of gene into mouse genome; conditional - targeted through homologous recombination
alk phos
Defective neutral amino acid transporter on renal and intestinal cells; AR
HMP shunt (to NADPH) and glycogenolysis (as glucose 1 phosphate)

2. How do you treat fructose intolerance?
Decrease intake of fructose and sucrose (fructose and glucose)
Glycerol and propionyl coA; lactate and alanine
Glycolysis - fatty acid synthesis - HMP shunt - protein synthesis (RER) - steroid synthesis (SER) - glycogenolysis - glycogenesis
Pyruvate dehydrogenase; ATP - NADH - acetyl CoA

3. What do statins inhibit?
HMG CoA reductase
Dark connective tissue - brown pigmented sclera - debilitating athralgias; homogentistic acid builds up and can be toxic to cartilage
Gets converted to glycerol 3 P Which is then converted to DHAP Which then can enter glycolyis
CPSII

4. What converts pyruvate to Acetyl CoA ? What allosterically downregulates this enzyme?
DHF
1) Dry beriberi: peripheral neuropathy demyelination (symmetricul muscle wasting) 2) Wernicke: ataxia - confusion - nystagmus - mammilary body and medial dorsal nuc of thalamus hemorrhage 3) Korsakoff: anterograde and retrograde amnesia - demyelinati
Pyruvate dehydrogenase; ATP - NADH - acetyl CoA
Highly condensed forms of DNA; mitosis; neoplasia - highly dividing and thus hyperchromatic

5. What causes fat soluble vitamin deficiencies?
Alpha 1 -4 linkages; alpha 1 -6 linkages
It is apart of thiamine pyrophosphate Which is a cofactor enzymes involved in ATP production: pyruvate dehydrogenase (glycolysis) - alpha ketoglutarate dehydrogenase (TCA cycle) - transketolase (HMP shunt) - branched chain AA dehydrogenase
high Vmax
Malabsorption problems (CF and sprue) and mineral oil intake

6. What is the deficient enzyme in Von Gierkes disease? What are the findings?
Glutamate and aspartate; CO2 provides an ketone
Arginine
Severe fasting hypoglycemia - very high levels glycogen in liver - high blood lactate - hepatomegaly
to ions

7. What converts dihydrobioterin back to tetrahydrobioterin factor? What energy carrier is used? in What reactions is this cofactor used?
Hexosaminidase A; GM2 ganglioside
A topoisomerase II
Dihydropterin reductase; NADP+; phenylalanine to tyrosine and tyrosine to DOP
cell cycle events

8. CFTR channel ______ Cl into sweat - secretes NaCl in luminal secretions
Cells that are stable go in and out of the G1 to G0 Which is the stable/quiescent (can go into replication but dont) type of cells; cells that stay in G0 are permanent and can not enter G1
reabsorbs
Carnitine acyltransferase I
Defective neutral amino acid transporter on renal and intestinal cells; AR

9. What aa becomes essential with phenylketonuria?
fructose
Tyrosine
Cytosine to uracil
Vit K; neonates have sterile intestine and can not synthesize vit K

10. On Which side of the cytoplasm is the ATP site in the Na+K+ ATPase pump? is a phosphorylated pump active or inactive?
Cytoplasmic side; active
Alpha ketoacids in the blood; alpha keto acids in the urine/poo smells of maple syrup/burnt sugar
Tyrosine
7 dehydrocholesterol

11. in Fabrys - What accumulates in tissue because fibroblasts express the def enzyme highly?
Orotic acid in blood and urine (from pyrimidine synthesis) - decreased BUN - and symptoms of hyperammonemia
Ceramide trihexoside
Phenylketones that build up in phenylketonuria
Schilling test

12. What cofactor is necessary for conversion of dopamine to NE? NE to epinephrine?
Meconium ileus and failure to thrive; negative serum immunoreactive trypsin
SAM; Vit C
Multiple bone fractures - blue sclerae - hearing loss (abdnormal middle ear bones!) - dental imperfections - retarded wound healing
to ions

13. What are chromosomes? during What part of the cell cycle are they found? in What pathologic state would you see a lot of chromosomes?
3'--> 5'; prokaryote only
Abs against type IV collagen; pulmonary and glomerular capillaries
Highly condensed forms of DNA; mitosis; neoplasia - highly dividing and thus hyperchromatic
TPP (thiamine pyrophosphate)

14. What disease is associated with chromosome 21?

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15. What toxins prevent elongation in translation in humans (by binding 40s)?
Back up of pyruvate and alanine and thus lactic acidosis occurs; can be d/t alcoholism (vit B1 def)
Shiga toxin and ricin (protein in castor beans)
= q
Gets converted to pyruvate via the Cori cycle in the liver

16. How many bonds does a G- C connection have? What type of bonds? how about a A- T?
When one allele on a gene is only expressed and the other is methylated/inactivated; during gametogenesis one of the alleles is methylated because the gamete from the other sex will provide those characteristics for the child
3 hydrogen bonds; 2 hydrogen bonds
Constitutive - random insertion of gene into mouse genome; conditional - targeted through homologous recombination
ATP and biotin

17. From What aa does GABA come from? with What enzyme? and What vit cofactor? What does GABA stand for?
succinyl coA (and then to OXA)
Glutamate; glutamate decarboxylase; B6; gamma aminobutyrate
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
Glycogen phosphorylase

18. How do you differentiate causes of megaloblastic anemia?
Later
Orotic aciduria megaloblastic anemia can not be corrected with folic acid or vit B12
Carbomyl phosphate synthetase I
both liver and muscle (unlike glucagon)

19. What happens to S adenosyl methionine after it donates a methyl? What enzyme regenerates methionine? What two factors are necessary to do this? who gives the methyl?
Meconium ileus and failure to thrive; negative serum immunoreactive trypsin
Aldose reductase in the lens converts built up galactose to galactitol Which is osmotically active
Glycolysis - fatty acid synthesis - HMP shunt - protein synthesis (RER) - steroid synthesis (SER) - glycogenolysis - glycogenesis
S adenosyl homocysteine; homocysteine transferase; B12 and THF; M- THF

20. Heterozygous females in X linked recessive disease...
UV nonionizing radiation causing thymidine dimers
can be d/t lyonization
Lys and arg
Exons

21. Coris glycogen storage disease has...
Ornithine transcarbomyalse; orotic aciduria; X linked recessive; autosomal recessive
normal lactate levels
D2 (ergocalciferol) - D3 (cholecalciferol) - D3 (cholecalciferol)
Ouabain

22. What are the cofactors required for pyruvate dehydrogenase?
Glutamate; glutamate decarboxylase; B6; gamma aminobutyrate
Imprinting Dx's: prader Willi and Angelmann
TPP (from B1- pyrophosphate) - FAD (B2) - NADH2 (B3) - CoA (B5 - panothenate) - Lipoic Acid
Carnitine shuttle

23. What reactions occur in the mitochondrial matrix? mit inner membrane?
TCA cycle - FA oxidation - acetyl coA production; ox phos
Tyrosine
No
GTP: gluconeogenesis - OXA to phosphoenolpyruvate

24. What is the source of Biotin?
Bacterial synthesis in the intestine
Glutamine - aspartate - glycine; aspartate
Galactose
Lactating mammary glands - adrenal cortex (sites of fatty acid or steroid synthesis) - RBCS (for glutathione reduction for FR injury)

25. How do you treat niacin deficiency do to corn based diet?
Citrate is positive on it - AMP and fructose 2 -6 bisphosphate is negative on it; gluconeogenesis
Lime and CaCO3 to displace the niacin
Zero order; increased production of NADH no matter the amount of end products produced
Flouroquinolones

26. Why can't muscle participate in gluconeogenesis - What key enzyme does it lack?
By activating protein phosphatase Which both dephosphorylates glycogen phosphorylase (deactivating) and dephosphorylates glycogen synthase (activating it )
5' capping (with 7 methylguanosine) - 3' adenylation - splicing of introns; nucleus
Glucose 6 phosphatase
CPSII

27. What is the function of B2? What can cause deficiency?
Metanephrine
Downs (21) - Edwards (18) - Pataus (13); also in increasing severity
Riboflavin makes up FAD and FMN both used in redox reactions; vegans
By addition of mannose 6 phosphate to proteins in the Golgi

28. Glycogenolysis ________ maintains blood sugar
can be d/t lyonization
in hepatocytes
Schilling test
Tay Sachs - Niemann Picks - Gauchers

29. Which TCA cycle enzyme is found in the inner mit membrane? Where are the rest found?
Galactose
Chromosomal imbalance (downs for ex)
S - adenosyl methionine
Succinate hydrogenase; matrix

30. What type of disease is I cell disease? What is deficient?
TPP (thiamine pyrophosphate)
A lysosomal storage disease; mannose 6 phosphate can not be added to lysosomal proteins
Lactose (glucose and galactose)
Hypophosphatemia rickets and Alports

31. What enzyme is deficient in Fructose intolerance? What is the result of the enzyme def? What are the symptoms?
Aldolase B; Fructose 1 phosphate accumulates Which uses up Phosphate and the result is inhibition of gluconeogenesis without phosphate; hypoglycemia - jaundice - cirrhosis - vomitting
When the disease manifestation depends on from who the gene deletion was inherited from d/t methylation that occurs; Prader Willi and AngelMann
Endonucleases - exonucleases - DNA polymerase - DNA ligase
Tay Sachs - Niemann Picks - Gauchers

32. What is the purpose of the PCR?
Nucleoside is just the ribose plus the base; nucleotide is the ribose plus the base but the phosphate bond
Amniocentesis karyotyping
Vomiting - rice water stools - garlic breath
To amplify a specific desired fragment of DNA

33. What are two examples of X linked Autosomal Dominant disease?
Hypophosphatemia rickets and Alports
AD (trinucleotide expansion is not a mode of inheritance)
Double strand breaks; nonhomologous end joining
Lactic acidosis and ketoacidosis

34. What causes gout in alcoholism?
Aminoglycosides and tetracyclines
Nacetyl cysteine; breaks disulfide bridges of mucus plugs
Acetyl CoA carboxylase
Increased acid production competes with uric acid for excretion

35. What happens in an aminotransferase reaction
GTP: gluconeogenesis - OXA to phosphoenolpyruvate
Gets converted to glycerol 3 P Which is then converted to DHAP Which then can enter glycolyis
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
Phospholipids - sphingolipids - glycolipids

36. Where are cilia found? What symptoms are associated with kartageners syndrome?
Via fructose 1 P by fructokinase in the liver; as glucose 1 by being GALT- ed ;)
No
Cystathionine (which then goes to cysteine) via cystathionine reductase; B6
In the reproductive tract and in the resp tract; infertility - bronchiectasis and recurrent sinusitis - associated with situs inversus

37. High blood levels of fructose - galactose - glucose can result in conversion to osmotically active alcohol forms by...
is asymmetric
paternal
Using serine (which turns into glycine after reaction)
aldose reductase

38. Where is there decreased hydrogen concentration in the mit? increased?
Ouabain
Either salvage by HGPRT and PRPP or conversion to xanthine and then uric acid by xanthine oxidase
Matrix; intermembrane space
all are diseased!

39. From What aa does glutathione come from?
Adenosine deaminase deficiency; Severe combined immunodeficiency disease
A 16C FA! the product in FA synthesis; saturated; no double bonds
Macroorchitism - big ears - big jaw - mental retardation (2nd mcc!) - mitral valve prolapse - long face
Glutamate

40. What is the primary source of nitrogen in the urea cycle
Phosphoenolpyruvate; pyruvate kinase; fructose 1 - 6 BP; ATP - alanine
Glycolysis and aerobic respiration
Glutamate
When one allele on a gene is only expressed and the other is methylated/inactivated; during gametogenesis one of the alleles is methylated because the gamete from the other sex will provide those characteristics for the child

41. What are rotenon - CN - antimycin A - CO?
Because cells of mononuclear phagocytic are esp rich in lysosomes
Downs
Vit K; neonates have sterile intestine and can not synthesize vit K
Block Electron transport

42. What are the findings in ornthinie transcarbamoylase?
fat
NADH - NADPH - FADH2
Orotic acid in blood and urine (from pyrimidine synthesis) - decreased BUN - and symptoms of hyperammonemia
Trimethroprim inhibits bacterial dihyrofolate reductase

43. What two enzymes are the apart of the same complex? What dictates What form they are in? and What dictates that?
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
LDL; LDL oxidize atherosclerosis
Fructose bisphosphatase 2 and Phosphofructokinase 2; phosphorylation by protein kinase A; inuslin/glucagon ratio
alk phos

44. GMP is degraded to guanosine Which is degraded to guanine. What two options are next? by What enzymes?
Either salvage by HGPRT and PRPP or converstion to xanthine and then to uric acid by xanthine oxidase
With glucose 6 phosphatase; Von Gierkes disease
Liver mit and liver cytosol
Microcephaly - mental retardation - high pitched mewing/crying - epicanthal folds - cardiac abnormalities; microdeletion on short arm of chrom 5

45. 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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46. Findings for McArdle's disease? deficient enzyme?
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
both liver and muscle (unlike glucagon)
Presence of normal and mutated mtDNA (like mosaicism except in mit)
Glucose 6 phosphatase

47. other than in the kidney - Where else are gluconeogenesis enzymes found?
As BUN (blood urea nitrogen)
Alanine aminotransferase (ALT) and aspartate aminotransferase (AST); alanine to pyruvate and aspartate to oxaxloacetate
Consumption of bear liver - and overtreatment isoretinoin; constitutional symptoms and alopecia and skin changes
Kidney and intestinal epithelium

48. What 3 genetic disorders can cause Marfanoid habitus?
Liver; P450 hydroxylation
Marfans - homocystinuria - MEN 2B
Collagen; fibroblasts; 4; ECM
Lack of lactase (brush border enzyme) so can not break down lactose (a dissaccharide of galactose and glucose); African Americans and Asians

49. Where does gluconeogenesis occur?
Karyotyping
Liver mit and liver cytosol
Schwann cells - lens - retina - kidneys because they only have aldose reductase and no sorbital dehydrogenase
By addition of mannose 6 phosphate to proteins in the Golgi

50. What is the first step in the respiratory burst/oxidative burst? in What disease is this deficient?
Mitochondria in liver; HmG CoA synthetase; amino acids and beta oxidation of FA
tyrosine hydroxylase
Ouabain
NADPH oxidase; CGD

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USMLE Step 1 Biochemistry

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