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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 is the difference between methotrexate and trimethroprim?
Nacetyl cysteine; breaks disulfide bridges of mucus plugs
A topoisomerase II inhibitor; because cancer cells use II more so than healthy cells
Trimethroprim inhibits bacterial dihyrofolate reductase
By activating adenylyl cylase Which increases cAMP Which increases levels of protein kinase A Which activates glycogen phosphorylase kinase; epinephrine

2. What is significant about 50S/60S ribosomal unit?
It contains the peptidyltransferase activity (for elongation of protein
Helps make Vit K and antioxidant
CCA and chemically modified bases Where amino acid is covalently bonded
Inbit Na+K+ ATPase; Cardiac glycosides

3. What can cause a vit C deficiency?
Differentiation of cells - vision in reduced light (retinol makes up rhodopsin cells) - antioxidant
Mucus secreting goblet cells - ab secreting plasma cells - nt secreting neurons
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)
Smoking (FR takes up all the antioxidants); diets lacking fruit

4. What type of polymerase is primase?
In the reproductive tract and in the resp tract; infertility - bronchiectasis and recurrent sinusitis - associated with situs inversus
Protein Kinase A inhibits pyruvate kinase also ATP and alanine have negative effects on pyruvate kinase
DNA dependent RNA polymerase
Marfans - homocystinuria - MEN 2B

5. What are the common symptoms of B vitamin deficiencies?
Dermatitis - glossitis - diarrhea
Cycloheximide
Abetalipoproteinemia and increased G3P production from all the carbs!
Sphingomyelinase; sphingomyelin

6. What is the precursor in the skin that the sun turns to D3?
anabolic processes
Inosine/IMP; adenosine deaminase
jaundice
7 dehydrocholesterol

7. What does the brain do with ketone bodies?
Down's Syndrome
Metabolizes them with transferase enzymes to acetyl coA
Osteomalacia is vit D def seen in adults and causes bowed legs and results in increased fractures
alk phos

8. What is def of Vit E caused by?
Fat malabsorption (CF) and abetalipoproteinemia (damage takes up all antioxidants)
Glycogen and FFA oxidation - glucose conserved for final sprinting!
By epoxide reductase in the liver; coumadin; intestinal flora and leafy vegetables
Sulfonamides - primaquine - dapsone - antiTB drugs - fava beans - LDL; RBCs

9. What does the passage of electrons in the ETC result in?
1 day; adipose release of FFA - hepatic gluconeogenesis from peripheral tissue lactate and alanine and from adipose tissue glycerol and propionyl coA
Formation of a proton gradient that is then used to create ATP through ATP synthase
Smoking (FR takes up all the antioxidants); diets lacking fruit
Prolonged Ab use

10. What reactions occur in the mitochondrial matrix? mit inner membrane?
TCA cycle - FA oxidation - acetyl coA production; ox phos
SER and mitochondria; NADPH
Release alk phos
Do agarose gel eletrophoresis to separate the different sizes of the PCR products (sizes id the the fragment)

11. The three steps of PCR
Conversion to UDP glucose via UDP glucose pyrophosphorylase; glycogen synthase (rate limiting step )
Both add phosphate; phosphorylase uses no ATP and inorganic phosphate
1. denature by heating 2. during cooling anneal with DNA primers 3. elongation - add heat stable DNA polymerase 4. repeat
LDL; LDL oxidize atherosclerosis

12. Which amino acids are basic? are they negatively or positively charged at body pH?
Arg - lys - his; positive except His has no charge at body pH
Folinic acid; does not require DHF
Multiple bone fractures - blue sclerae - hearing loss (abdnormal middle ear bones!) - dental imperfections - retarded wound healing
Branche aa alpha ketoacid dehydrogenase; thiamine; Isoleucine - leucine - valine

13. What does alcohol do to the P450 system?
Vincristine - vinblastine - and paclitaxel; the vinca alkaloids block formation of the spindle and paclitaxel prevents breakdown of the mT spindle
Induction in chronic alcoholism (increases tolerance); inhibition in acute alcoholism
7 dehydrocholesterol
Tay Sachs - Niemann Picks - Gauchers

14. What is the exception to degeneracy in the genetic code?
Def in Vit C; def in Vit D
Codons may encode same amino acid since there are only ~20 aa
Tyrosine
Methionine is only encoded by AUG

15. What is the rate limiting step in fatty acid synthesis? What four things inhibit this rate limiting step? What stimulates it?
Acidic
Reserve supply of glucose in the fasting state - liver gives to the blood and skeletal muscle uses for itself
Acetyl coA carboxylase converting Acetyl coA into malonyl coA; increased AMP - glucagon - epinephrine - palmitate; citrate
Glutamate; glutamate decarboxylase; B6; gamma aminobutyrate

16. Which anticancer drugs act on MT? how are they different?
calorie deficient
Vincristine - vinblastine - and paclitaxel; the vinca alkaloids block formation of the spindle and paclitaxel prevents breakdown of the mT spindle
Coenzyme A and lipoamides
By activating adenylyl cylase Which increases cAMP Which increases levels of protein kinase A Which activates glycogen phosphorylase kinase; epinephrine

17. How many enzymes does the pyruvate dehydrogenase complex have? and How many cofactors? Where is it located? to What enzyme is it similar to?
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
both liver and muscle (unlike glucagon)
3; 5; mit matrix; alpha ketoglutarate dehydrogenase complex
Aromatic; musty body odor

18. What disease is associated with chromosome 3?
VHL (Von Hippa Lindau)
DiGeorge and Velocardiofacial; deletion on chromosome 22q11 results in aberrant dev of 3rd and 4th branchial pouches; digeorge has thymic and parathyroid prob - velocardiofacial has palate and facial instead
Lactose (glucose and galactose)
Liver; P450 hydroxylation

19. How is THF converted to N5N10 Methylene?
Beckers is less severe (instead of deletion is either defect or deficiency) with later presentation
Using serine (which turns into glycine after reaction)
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
Arg and his

20. give an example of how phenylketonuria is pleiotropic.
Pernicious anemia; older - mentally slow european woman who is anemic and icteric with a smooth shiny tongue and has broad based shuffling gait
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
FA coA synthetase combines coA with FA to make acyl coA
Also have fair skin - blond hair - blue eyes - eczema

21. Where does the HMP (pentose phosphate pathway) occur other than in the liver?
Lactating mammary glands - adrenal cortex (sites of fatty acid or steroid synthesis) - RBCS (for glutathione reduction for FR injury)
Complex I - Complex II (lower energy)
succinyl coA (and then to OXA)
Increased concentration of Cl ions in sweat test

22. What is the deficient enzyme in Tay Sachs disease? accumulated Substrate?
Hexosaminidase A; GM2 ganglioside
Glutamine PRPP amidotransferase
normal lactate levels
Lactose (glucose and galactose)

23. Name two proapoptotic/tumor suppressor genes. How do they each act?
Preprocollagen - procollagen - tropocollagen - collagen
plasma membrane
P53 and Rb; p53 activates BAX Which inhibits BCL2 When there is DNA damage - Rb phosphorylation allows progression into S phase
ATP and dATP

24. What are the findings in Krabbes disease?
Oral uridine administration; provides nucleosides and provides feedback inhibition
Microvilli - muscle contraction - cytokinesis - adherens junctions
Very Poor Carb Metabolism (Von Gierke - Type 1 - Pompes - Type II - Cori's - Type III - McArdles disease - Type IV)
Peripheral neuropathy - developmental delay - optic atrophy - globoid cells

25. What are the glucogenic amino acids?
Carbomyl phoshpate synthetase I
Phenylaline
Met - arg - val - his
Inhibits lipoic acid; lipoic acid is a cofactor for pyruvate dehydrogenase

26. What is achondroplasia? What type of inheritance?
Osteoblasts; de phosphorylates pyrophosphate Which inhibits bone mineralization; bone mineralization
Tay Sachs - Niemann Picks - Gauchers
Cell signaling defect in FGF receptor resulting in dwarfism and short limbs with normal head and trunk; AD
Orotic aciduria megaloblastic anemia can not be corrected with folic acid or vit B12

27. Which amino acids are required during periods of growth?
32 or 30; malate aspartate shuttle or glycerol 3 phosphate shuttle; heart/liver and muscle
Glycine; heme
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
Arg and his

28. What is used in a marathon?
Fatty acid (acyl coA) oxidation to acetyl coA; increase in dicarboxylic acids - and decreased glucose and ketones
Glycogen and FFA oxidation - glucose conserved for final sprinting!
Fructose bisphosphatase 2
TPP (from B1- pyrophosphate) - FAD (B2) - NADH2 (B3) - CoA (B5 - panothenate) - Lipoic Acid

29. What is there on increased risk of with Downs?
All and Alzheimers
Yes! congenital simply means 'born with'- hereditary is familial
Colchine
NADPH oxidase - superoxide dismutase - myeloperoxidase; in the phagolysosome; HOCl (bleach); kills!

30. Why does fa degradation occur in the mitochondria? What does FA degradation produce? What is the fate of this product?
Arylsulfatase A; cerebroside sulfate
Because that is Where the products will be consumed; acetyl coA; TCA cycle of ketone body production
DNA dependent RNA polymerase
Homocystinuria (homocysteine damages endothelial cells); alkaptonuria (tyrosine degradation); homocystinuria

31. What enzyme is necessary for branched aa degradation? What cofactor? What are the branched aa?
FA coA synthetase combines coA with FA to make acyl coA
Branche aa alpha ketoacid dehydrogenase; thiamine; Isoleucine - leucine - valine
When the amino acid is covalently binded to its corresponding tRNA; Aminoacyl tRNA synthetase; ATP; translation
Downs

32. What are flagella made of?
Glucose 6 phosphatase
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
Nucleoside is just the ribose plus the base; nucleotide is the ribose plus the base but the phosphate bond
MT!

33. What is the job of S adenosyl methionine? What is it made of ? give an example of a reaction it participates in.
To transfer methyls; a methionine and an ATP; norepinephrine plus a methyl makes epinephrine
Phosphorylation of glucose
Formation of a proton gradient that is then used to create ATP through ATP synthase
FISH

34. What enzyme converts DHF to THF? What vitamin cofactor is used to transfer a methyl from an amino acid to DHF? What amino acid is that?
Double strand breaks; nonhomologous end joining
Pigmented iris hamartomas
Dihydrofolate reductase; vitamin B12; homocysteine
Pataus

35. What ensures that during gluconeogenesis all the ATP wont be used in glycolysis as youre producing it?
Protein Kinase A inhibits pyruvate kinase also ATP and alanine have negative effects on pyruvate kinase
Dihydrofolate reductase; vitamin B12; homocysteine
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
Niacin/Nicotinic acid; NAD+ and NADH used in redox reactions - and catabolic and anabolic reactions; B6; tryptophan

36. Give an example of codominance.
Blood group types
teratogenic
Glycerol and propionyl coA; lactate and alanine
Gauchers; hepatosplenomegaly - aseptic necrosis of the femur - bone crises - Gauchers cells - crumpled tissue paper looking (fibrillar appearing macrophages in liver - spleen - b.m.)

37. With What type of inheritance is variable expression and incomplete penetrance associated with? Give an example of each.
Metanephrine
1 for all three classes of RNA; Rifampin; TB
Its the ribose; its made from PRPP synthetase using ribose 5 P from the HMP shunt
Autosomal Dominant; NF1 and mit. diseases (variable expression) - ?

38. Which amino acid is the most basic?
Adrenal medulla
Met - arg - val - his
Increased blood levels of ammonia - decreased BUN
Arg

39. Which antioxidant does cigarette smoke injury eat up? Which antioxidant does LDL injury eat up?
CPSII
Phosphofructokinase 1
Vit C; Vit E
N linked oligosaccharides

40. What generally results after Vitamin B1 deficiency? T/F treat b1 def - with glucose.
7 dehydrocholesterol
ATP depletion and highly aerobic tissues (heart and brain) are affected first; false - glucose infusion worsens
Increased blood levels of ammonia - decreased BUN
Acetyl coA; gluconeogenesis

41. What is the most abundant protein in the body? From What cell type is it made? How many types are there? Where is it found (generally)?
(cofactor for 100+ enzymes!) needed for zinc fingers transcription motif - collagen synthesis - alcohol metabolism
Down's Syndrome
Isocitrate dehydrogenase
Collagen; fibroblasts; 4; ECM

42. other than bone resorption to release Ca and phosphate - What is the ultimate point of increasing the serum Ca2+ and phosphate?
To increase serum Ca and phosphate for bone deposition
Cysteine; tyrosine
reabsorbs
AD

43. What are the three results that come from increased NADH in alcohol metabolism?
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
5' end; 3' end
O2 consumption (last electron acceptor) gets consumed also
Def in Vit C; def in Vit D

44. What are the findings in alkaptonuria? why do they occur?
Dark connective tissue - brown pigmented sclera - debilitating athralgias; homogentistic acid builds up and can be toxic to cartilage
Shiga toxin and ricin (protein in castor beans)
Griseofulvin
Alpha ketoacids in the blood; alpha keto acids in the urine/poo smells of maple syrup/burnt sugar

45. What is dysgeusia?
Inability to taste
Alpha ketoglutarate; no TCA cycle intermediates
Mental and growth retardation; seizures - fair skin - eczema - musty body odor; neurotoxic ketones and acids stimulate vomitting - can lead to pyloric stenosis
1 day; adipose release of FFA - hepatic gluconeogenesis from peripheral tissue lactate and alanine and from adipose tissue glycerol and propionyl coA

46. How does insulin both deactivate glycogenolysis and activates glycogenesis at same time?
Galactokinase deficiency and classic galactosemia; classic galactosemia
By activating protein phosphatase Which both dephosphorylates glycogen phosphorylase (deactivating) and dephosphorylates glycogen synthase (activating it )
Glucose 6 phosphate dehydrogenase
show disease

47. Fatty acid coA synthetase is used in FA...
degradation
Adrenal cells (steroid synthesis) - hepatocytes (p450 detox)
3'--> 5'; prokaryote only
Hydroxurea; deoxygenates ribose

48. The result of dephosphorylation of PFK2 is...
activation of PFK1
Neimann pick and tay sachs
Insertion or deletion of a number of nucleotides indivisible by 3; tay sachs - duchennes MD
Liver; berry aneurysms (d/t hypertension!) - and mitral valve prolapse

49. What functions are completed in the SER?
to ions
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
Block Electron transport
Steroid synthesis and detoxification

50. How is excess nitrogen taken care of?
enzymatic
Glycerol and propionyl coA; lactate and alanine
Excreted from the kidney in the form of urea in hepatocytes
Phenylaline

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USMLE Step 1 Biochemistry

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