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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. What amino acids are required in purine synthesis? Which of these does pyrimidine synthesis require also?
pleiotropic
Glutamate
Back up of pyruvate and alanine and thus lactic acidosis occurs; can be d/t alcoholism (vit B1 def)
Glutamine - aspartate - glycine; aspartate

2. What is the treatment for cystinuria?
Acetazolamide to alkalinize the urine
DHF
Hemolytic anemia with a blood smear with heinz bodies and bite cells (phagocytic removal of heinz bodies)
When different combination of exons get combined together; in Beta thallasemia

3. Which oxidant is most attributable to damage of RBCs?
Stored ATP - creatine phosphate - anaerobic glycolysis; as distances increase - ATP is obtained from additional resources; above plus ox phos
LDL; LDL oxidize atherosclerosis
Renal failure - decreased sun - fat malabsorption - chronic liver disease - p450 induction - hypoparathyroidism (activates 1 alpha hyrdoxylase)
Kidney; 1 alpha hydroxylase; Parathyroid hormone

4. How do you differentiate causes of megaloblastic anemia?
Orotic aciduria megaloblastic anemia can not be corrected with folic acid or vit B12
Schilling test
Phenylalanine
Carnitine acyltransferase I

5. What enzyme is deficient in maple syrup urine disease?
Defective neutral amino acid transporter on renal and intestinal cells; AR
3 NADH - 1 FADH2 - 2 CO2 - 1 GTP = 12 ATP/acetyl CoA
Branched alpha ketoacid dehydrogenase
Assist in spontaneous refolding of proteins

6. Homocysteine gets turned into methionine by...
homocysteine transferase
Smoking (FR takes up all the antioxidants); diets lacking fruit
Telomerase; an RNA polymerase that uses a RNA template to add DNA to the end of shortening 3' ends
anabolic processes

7. What aa does melanin come from? melatonin?
An allosteric regulator; fructose 1 phosphate; phosphofructokinase 2
Preprocollagen - procollagen - tropocollagen - collagen
B12 and folate; B12; washed out
Tyrosine; tryptophan

8. HMG coA synthase; cholesterol synthesis
Lysine and arginine; basic
Uses HMG coA reductase
No; increased mutation rate!
Progressive neurodegeneration (spasticity d/t UMN disease) - hepatosplenomegaly - cherry red spot on macula - foam cells

9. What are the three results that come from increased NADH in alcohol metabolism?
Conversion to UDP glucose via UDP glucose pyrophosphorylase; glycogen synthase (rate limiting step )
Dark connective tissue - brown pigmented sclera - debilitating athralgias; homogentistic acid builds up and can be toxic to cartilage
Pyruvate to lactate (results in lactic acidosis and decreased pyruvate for oxaloacetate for gluconeogenesis) - increased production of glycerol 3 phosphate from DHAP for TG synthesis; and increased betahydroxybutyrate ketoacid production from increas
By activating protein phosphatase Which both dephosphorylates glycogen phosphorylase (deactivating) and dephosphorylates glycogen synthase (activating it )

10. in What disease are there antibodies to snRNPs?
Tyrosine
Lupus
Cytoplasmic side; active
Bendy bones seen in children due to decreased Vit D; craniotabes - skeletal deformities - growth retardation - rosary

11. list three uncoupling agents
Liver mit and liver cytosol
Uses HMG coA reductase
Type I collagen; AD
2 -4 dinitrophenol - aspirin - and thermogenin (in brown fat)

12. What is the effect of defiiency of VIt E?
Karyotyping
Histidine; B6
Schilling test
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

13. What does ornithine transcarbamyolase do to carbamoyl phosphate?
Decreased would healing; microcytic anemia (d/t decreased iron [not absorbable]; macrocytic anemia (d/t Fh4 not reduced)
Converts it to citrulline
Some viruses
Imprinting Dx's: prader Willi and Angelmann

14. What is the confirmation of Downs?
Increased concentration of Cl ions in sweat test
kidney
SAM; Vit C
Amniocentesis karyotyping

15. What disease is associated with chromosome 21?

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16. What is the order of cell cycle phases? What happens in each phase?
Bacteria produce lactate acid and gases from lactose and produce acidic diarrhea and bloating
As BUN (blood urea nitrogen)
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)
Liver and skeletal muscle

17. Where are ketone bodies made? What is the rate limiting enzyme? What are they made from?
Type I collagen; AD
Mitochondria in liver; HmG CoA synthetase; amino acids and beta oxidation of FA
Uric acidemia; HGPRT; purine salvage
Arginne

18. What is the rate limiting step in cholesterol synthesis? What does it do?
HMG CoA reductase; converts HMG CoA to mevalonate
Vit C; Vit E
Exons
allosteric regulation

19. What is imprinting disease? Give 2 examples
When the disease manifestation depends on from who the gene deletion was inherited from d/t methylation that occurs; Prader Willi and AngelMann
AD
HMG CoA reductase; converts HMG CoA to mevalonate
'9+2' 9 pairs of microtubules connected by an axonemal dynein ATPase in a cylinder with 20 MTs in the center; the connecting ATPase allows bending and differential sliding

20. What else decreases production of oxaloacetate in alcoholism other than decreased pyruvate?
Retinol; Beta carotenes (dark and yellow vegetables)
Tyrosine
Chediak Higashi Syndrome; decreased phagocytosis results in recurrent pyogenic infections - partial albinism - and peripheral neuropathy
Oxaloacetate gets shifted to malate d/t excess NADH

21. What is the function of Vit K?
Catalyze gamma carboxylation of glutamic acid residues on coag factors so they can bind to Ca complex
CNS defects - mental retardation - death - feeding difficulties - vomiting - hypoglycemia; thiamine
Biotin; rate limiting step in fA synthesis - acetyl coA into malonyl coA
Bacterial synthesis in the intestine

22. What is the difference between the promoter and enhancer? in What process are they used for?
25 OH D3; 1 -25 (OH)2 D3 (Calcitriol)
Kidney and intestinal epithelium
Promoter binds RNA Pol II and is very close (25 bases) to initiation site and is AT rich; enhancer/silencers bind TFs only - can be located close or far and regulate gene expression but not specifically initiation; transcription
(cofactor for 100+ enzymes!) needed for zinc fingers transcription motif - collagen synthesis - alcohol metabolism

23. Breast milk has a little of...
Vit D and K
Inosine/IMP; adenosine deaminase
Consumption of bear liver - and overtreatment isoretinoin; constitutional symptoms and alopecia and skin changes
Mental retardation - obesity - short stature - hypogonadism - hypotonia - hyperphagia; father

24. Is PRPP added later or first in pyrimidine synthesis?
Later
Conversion of pyruvate to Acetyl CoA; acetyl coA - CO2 - and NADH (one of each per pyruvate)
30S ribosomal RNA (small one)
Marfans - homocystinuria - MEN 2B

25. other than succinate dehydrogenase What other enzymes are found in the inner mit membrane?
Helps make Vit K and antioxidant
activation of PFK1
Phenylalanine
ATP synthase - ATP- ADP translocase - ETC

26. 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
UV nonionizing radiation causing thymidine dimers
Ouabain
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

27. Which water soluble vitamins get stored in the liver with the fat soluble vitamins? Which has a greater pool of storage in the liver? Where do all the other water soluble vitamins go?
Bone - Skin - Tendon - dentin - fascia - cornea - late wound repair
Glycosylases (only remove the base - cut the bond between ribose and base)
Double strand breaks; nonhomologous end joining
B12 and folate; B12; washed out

28. What causes the pathology in I- cell disease? What does I cell sidease
Microcephaly - mental retardation - high pitched mewing/crying - epicanthal folds - cardiac abnormalities; microdeletion on short arm of chrom 5
Dihydropterin reductase; NADP+; phenylalanine to tyrosine and tyrosine to DOP
The lack of lysosomal proteins (are secreted - default from RER!) results in macromolecules accumulating in lysosomes - inclusions cause pathology
Pulmonary - GI - reproductive (infertility in male due to absence of vas deferens and seminal fluid thickness)

29. What is the main hormonal regulation of glycogen?
Glucagon - insulin and epinephrine
In the reproductive tract and in the resp tract; infertility - bronchiectasis and recurrent sinusitis - associated with situs inversus
NADPH oxidase - superoxide dismutase - myeloperoxidase; in the phagolysosome; HOCl (bleach); kills!
2 -4 dinitrophenol - aspirin - and thermogenin (in brown fat)

30. What vesicular trafficking protein takes proteins from the Golgi to lysosomes? in between the golgi? from the plasma membrane to endosomes (receptor mediated endocytosis)?
Trinucleotide expansion diseases
long saturated fatty acids
All clathrin
Metaphase (all lined up); blood - bone marrow - amniotic fluid - placental tissue; used to diagnose chromosomal abnormalities (deletions - trisomies - sex chromosomal disorders)

31. Daughters of Xlinked dominant diseased fathers are...
all are diseased!
An anti cancer agent that is a topoisomerase II and creates nicks in DNA
Mental retardation - rocker bottom feet - micrognathia (small jaw) - low set ears - clenched hands with polydactyly - congenital heart disease
Lysine and arginine; basic

32. What happens in an aminotransferase reaction
Carbomyl phosphate synthetase I turns N acetyl glutamate into carbomyl phosphate and then ornithine transcarbomyalse turns carbomyl phosphate into citrulline (using ornithine)
Pernicious anemia; older - mentally slow european woman who is anemic and icteric with a smooth shiny tongue and has broad based shuffling gait
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
Flouroquinolones

33. What is the exception to the nonoverlapping/commaless aspect of the genetic code?
Electron transport inhibitors - ATPase inhibitors - Uncoupling agents (increase permeability of the membrane - loss of hydrogen gradient)
Some viruses
X linked recessive; deletion of the dystrophin gene (connects actin to the membrane glycoprotein) due to frame shift mutation - lack of dystrophin results in muscle breakdown Which gets replaced by fibrofatty tissue and collagen
Which dimerizes and activates a protein phosphatase Which de phosphorylates glycogen phosphorylase

34. in What organ is the storage form of D3 converted to its active form? What enzyme? What hormone stimulates this action?
As BUN (blood urea nitrogen)
2/3; LCAT (lecithin cholesterol acyltransferase)
severe
Kidney; 1 alpha hydroxylase; Parathyroid hormone

35. Where does a majority of galactose in our diet come from?
Met - arg - val - his
A ketone
Leucine and lysine
Lactose (glucose and galactose)

36. What are labile cells? Which cells are labile?
DOPA
Cells that never got into G0 and divide rapidly with a short G1; bone marrow - gut epithelium - hair follicles - skin
1) glycogenolysis (liver only contributes to plasma - muscle keeps it for itself) 2) gluconeogenesis
Downs (21) - Edwards (18) - Pataus (13); also in increasing severity

37. What three steps are included in RNA processing? Where does it occur?

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38. What is the deficient enzyme in Krabbes disease? accumulated Substrate?
Abs against type IV collagen; pulmonary and glomerular capillaries
Galactocerebrosidase; Galactocerebroside
to ions
Hereditary defect of renal tubular amino acid transporter for cysteine - ornithine - lysine and arginine in the PCT of kidnyes

39. _______ can not get inside cells
Inosine/IMP; adenosine deaminase
Indirect inhibtion of the Na+/Ca+2 exchange resulting in increased intracellular calcium Which increases cardiac contractility
fructose
Glycine; proline and lysine

40. What is the deficient enzyme in Metachromatic leukodystrophy disease? accumulated Substrate?
jaundice
Arylsulfatase A; cerebroside sulfate
Glucose 6 phosphate dehydrogenase
only yield acetyl coA

41. What carries CO2 as energy?
Biotin
VMA
AD; disorder of blood vessels that results in telangietasias - skin discolorations - and AVMs (arteriovenous malformations)
hexokinase (glucokinase only found in liver and beta cells of pancreas)

42. How does lactate contribute to gluconeogenesis?
Beckers is less severe (instead of deletion is either defect or deficiency) with later presentation
Gets converted to pyruvate via the Cori cycle in the liver
fat
Its When two acrocentric chromosomes combine their long arms (and short arms disappear); When the translocation is functional

43. What is the purpose of HMP shunt?
Downs
Each codon specifies only one amino acid
To provide NADPH since there is an abundance of glucose 6 phosphate; also yields ribose for nucleotide synthesis and glycolysis intermediates
Cafe au lait spots - optic pathway gliomas - lisch nodules - scoliosis - neural tumors

44. What is the metabolism of galactose like?
Retinol; Beta carotenes (dark and yellow vegetables)
can be d/t lyonization
Downs (21) - Edwards (18) - Pataus (13); also in increasing severity
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

45. What two enzymes are the apart of the same complex? What dictates What form they are in? and What dictates that?
'9+2' 9 pairs of microtubules connected by an axonemal dynein ATPase in a cylinder with 20 MTs in the center; the connecting ATPase allows bending and differential sliding
Cri du Chat
glycogen
Fructose bisphosphatase 2 and Phosphofructokinase 2; phosphorylation by protein kinase A; inuslin/glucagon ratio

46. Where does gluconeogenesis occur?
Complex I - Complex II (lower energy)
Pyruvate dehydrogenase; ATP - NADH - acetyl CoA
Sulfonamides - primaquine - dapsone - antiTB drugs - fava beans - LDL; RBCs
Liver mit and liver cytosol

47. Name two antineoplastic drugs that work by inhibiting the mitotic spindle by MT blocking.
Vincrstine (vinca alkaloids) and paclitaxel
Dark connective tissue - brown pigmented sclera - debilitating athralgias; homogentistic acid builds up and can be toxic to cartilage
Inability to transport LCFA into mitochondria results in their build up and causes weakness - hypotonia And HYPOKETOTIC HYPOGLYCEMIA
Neurons and RBCs

48. What converts tyrosine to DOPA? What cofactor is used?
Each codon specifies only one amino acid
Riboflavin makes up FAD and FMN both used in redox reactions; vegans
Tyrosine hydroxylase; tetrahydrobiopterin factor; NADP+
3; 5; mit matrix; alpha ketoglutarate dehydrogenase complex

49. In What syndrome is HGPRT deficient? What is the inheritance pattern?
Lesch Nyhan syndrome; X linked recessive
Fatty acid (acyl coA) oxidation to acetyl coA; increase in dicarboxylic acids - and decreased glucose and ketones
Pyruvate to lactate (results in lactic acidosis and decreased pyruvate for oxaloacetate for gluconeogenesis) - increased production of glycerol 3 phosphate from DHAP for TG synthesis; and increased betahydroxybutyrate ketoacid production from increas
Respiratory - seminal fluid - pancreatic - gallbladder; very thick without NaCl to pull water

50. What type exonuclease activity does DNA polymerase III have? in What organisms is it found?

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USMLE Step 1 Biochemistry

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