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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. How is Vit K activated? What drug blocks this? What is the source of Vit K?
Avidin in raw egg whites prevents their reabsorption; also antibiotic use; dermatitis - alopecia - glossitis - lactic acidosis - enteritis
AR
By epoxide reductase in the liver; coumadin; intestinal flora and leafy vegetables
Cytoplasmic side; active

2. What type of aa is phenylalanine? What implications does this have on the presentation of phenylketonuria?
fructose
Pyruvate! (not lactate - that is generate just in order to produce more NAD+)
Arginne
Aromatic; musty body odor

3. How do you differentiate causes of megaloblastic anemia?
Orotic aciduria megaloblastic anemia can not be corrected with folic acid or vit B12
Pyruvate (through OXA); lactate - alanine - malate (and all other TCA intermediates through OXA) - glycerol - odd chain fatty acids yield propionyl coA Which can be converted to succinyl coA and enter as TCA cycle
Decreased substrate for gluconeogenesis (pyruvate to lactate and not oxaloacetate); and pyruvate doing only lactic acidosis and not glycolysis
tubulin dimer

4. There are 2 GTP per...
3'--> 5'; prokaryote only
Neurons and RBCs
tubulin dimer
Chorea and spastic paresis

5. ____________ disorders are worse than fructose disorders.
Downs
30S + 50S= 70S
Branche aa alpha ketoacid dehydrogenase; thiamine; Isoleucine - leucine - valine
Galactose

6. What are the key intermediates in the TCA?
Loss of subcutaneous fat - tissue and muscle wasting (somatic muscle); visceral - not somatic
Mental retardation - excessive laughter - seizures - ataxia - 'happy puppet'
Citrate - Isocitrate - alpha ketoglutarate - Succinyl CoA - Succinate - Fumarate - Malate - Oxaloacetate
Folinic acid; does not require DHF

7. Walk through the process that adipose tissue makes triacylglycerol.
Succinate hydrogenase; matrix
H1
Collagen; fibroblasts; 4; ECM
Lipoprotein lipases take FA from VLDL and chylomicrons Which are then combined with glycerol 3 P (a carb intermediate) to make triacylglycerol

8. What happens in RBCs after their membranes get oxidized?
Ouabain
Hemolytic anemia due to heinz bodies Which are oxidized hemoglobin precipitating within the RBCs
Neurons and RBCs
Citrate shuttle in the inner mitochondrial membrane transports acetyl coA into the cytoplasm; liver; liver and adipose tissue

9. What are the two disorders of galactose metabolism? Which is worse?
Proteolytic processing (of water soluble parts to less soluble) to turn into troprocollagen and then crosslinking between hydroxylated lysine residues by lysyl oxidase to make collagen fibrils
Glycogen synthase
Galactokinase deficiency and classic galactosemia; classic galactosemia
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

10. What is the first enzyme step in glycogenolysis? What activates it ? inactivates?
Glycogen phosphorylase; phosphorylation and dephosphorylation
Phosphofructokinase 1; fructose 1 phosphate to fructose 1 -6 bisphosphate; enhancers: AMP - fructose 2 -6 Bisphosphate; downregulators: citrate - ATP
Urea cycle enzyme deficiency (mc: ornithine transcarbamoylase); liver disease - alcoholic cirrhosis and reye's
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

11. What carries CO2 as energy?
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
Biotin
Pyridoxine; converted to pyridoxal phsophate used in transamination reaction - decarboxylations - heme synthesis - niacin synthesis from trp - glycogen phosphorylase - cystathionine synthesis - NT synthesis
The lack of lysosomal proteins (are secreted - default from RER!) results in macromolecules accumulating in lysosomes - inclusions cause pathology

12. 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
Dynein - retrograde - kinesin - anterograde
Increased acid production competes with uric acid for excretion
Beta glucocerebrosidase; glucocerebroside

13. other than scurvy What else does vit C def cause? also - What kind of anemias and why?
Decreased would healing; microcytic anemia (d/t decreased iron [not absorbable]; macrocytic anemia (d/t Fh4 not reduced)
Heart - SKELETON - and EYES: arachnodactyly - long limbs - cystic medial necrosis of aorta resulting in aneurysms (and eventual dissection) - floppy mitral valve (prolapse and regurg!) - subluxation of lens
No protein!
AD: yes! most common presentation; very rare (700 mg/dl cholesterol level!)

14. why is there decreased immunity in kwashiokor and marasmus?
Pyruvate (through OXA); lactate - alanine - malate (and all other TCA intermediates through OXA) - glycerol - odd chain fatty acids yield propionyl coA Which can be converted to succinyl coA and enter as TCA cycle
No protein!
Alpha 1 -4 linkages; alpha 1 -6 linkages
Insertion or deletion of a number of nucleotides indivisible by 3; tay sachs - duchennes MD

15. How do you treat orotic aciduria? How does this work?
A topoisomerase II
Oral uridine administration; provides nucleosides and provides feedback inhibition
Mental retardation - flat faces - simean crease - epicanthal folds - ASD - duodenal atresia causing Hirshsprungs disease - redundant skin at the nape of neck - slanted palpebral fissures - hypotonia
This depends on if phosphofructokinase 1 is active and this depends on whether or not fructose 2.6 bisphophate is activating it or not - this can only occur if PFK 2 is on and not FBPase 2. then fructose 6 phosphate will participate in glycolysis. Ot

16. What are the glucogenic amino acids?
NADPH- reductive biosynthesis!
Met - arg - val - his
Mucus secreting goblet cells - ab secreting plasma cells - nt secreting neurons
Oral uridine administration; provides nucleosides and provides feedback inhibition

17. What is the other name for Vitamin B5? What is its function?
Pantothenate; essential comp of CoA (needed for acyl transfers) and fatty acid synthase
Fructokinase; benign - fructose in the blood and urine
Vit C deficiency; glossitis - anemia - weakened vessels - hemorrahges - hemarthroses - swollen gums - bruises -
Helps make Vit K and antioxidant

18. What are the last three steps of the respiratory burst? why do they need to occur? Where do they occur?
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
allosteric regulation
Its the ribose; its made from PRPP synthetase using ribose 5 P from the HMP shunt
Very Poor Carb Metabolism (Von Gierke - Type 1 - Pompes - Type II - Cori's - Type III - McArdles disease - Type IV)

19. What are the findings in Niemann Pick disease?
Progressive neurodegeneration (spasticity d/t UMN disease) - NO hepatosplenomegaly - cherry red spot on macula - lysosomes with onion skin - muscle weakness
Mental retardation - flat faces - simean crease - epicanthal folds - ASD - duodenal atresia causing Hirshsprungs disease - redundant skin at the nape of neck - slanted palpebral fissures - hypotonia
SAM; Vit C
Progressive neurodegeneration (spasticity d/t UMN disease) - hepatosplenomegaly - cherry red spot on macula - foam cells

20. What is imprinting?
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
Uracil; thymine
Neurons and RBCs
pleiotropic

21. What are the main clinical symptoms of Lesch Nyhan syndrome?
G6PD; provides malarial resistance; X linked recessive
Oxygen!
Downs
Self mutilation - aggression - mental retardation - gout - hyperuricemia - choreathetosis

22. Is PRPP added later or first in pyrimidine synthesis?
Later
Multiple bone fractures - blue sclerae - hearing loss (abdnormal middle ear bones!) - dental imperfections - retarded wound healing
Neurons and RBCs
Assist in spontaneous refolding of proteins

23. What type of disease is MELAS? What is the clinical presentation?
RER; free ribosomes
Bacteria produce lactate acid and gases from lactose and produce acidic diarrhea and bloating
Hereditary defect of renal tubular amino acid transporter for cysteine - ornithine - lysine and arginine in the PCT of kidnyes
Mit encephalopathy - lactic acidosis - stroke like episodes

24. What does 6 mercaptopurine do?
HMP shunt (to NADPH) and glycogenolysis (as glucose 1 phosphate)
6MP blocks de novo purine synthesis
AD: yes! most common presentation; very rare (700 mg/dl cholesterol level!)
CCA and chemically modified bases Where amino acid is covalently bonded

25. Which RNA polymerase synthesizes rRNA? where?
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
2/3; LCAT (lecithin cholesterol acyltransferase)
I; nucleolus
Increased homocysteine in the urine - mental retardation - osteoporosis - marfanoid habitus - and atherosclerosis

26. What are the glycogen storage disease? What they result in?
By activating protein phosphatase Which both dephosphorylates glycogen phosphorylase (deactivating) and dephosphorylates glycogen synthase (activating it )
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
Abnormal glycogen metabolism results in accumulation of glycogen in cells
Alpha ketoglutarate; no TCA cycle intermediates

27. What disease is associated with chromosome 18?
Edwards
By addition of mannose 6 phosphate to proteins in the Golgi
CCA and chemically modified bases Where amino acid is covalently bonded
high cholesterol

28. What can cause Biotin deficiency? What are the symptoms?
Autosomal Dominant; NF1 and mit. diseases (variable expression) - ?
Tremor (asterixis) - slurring of speech - decrease temperature - somnolence - vomitting - cerebral edema - blurring of vision - neural focal deficits
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
Avidin in raw egg whites prevents their reabsorption; also antibiotic use; dermatitis - alopecia - glossitis - lactic acidosis - enteritis

29. Which end carries the triphosphate? Which end of DNA makes the hydoxyl attack?

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30. 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
can be d/t lyonization
GALT
Inability to transport LCFA into mitochondria results in their build up and causes weakness - hypotonia And HYPOKETOTIC HYPOGLYCEMIA

31. What is the deficient enzyme in Fabry's disease? accumulated Substrate?
Alpha galactosidase A; ceramide trihexoside
Glycolysis - fatty acid synthesis - HMP shunt - protein synthesis (RER) - steroid synthesis (SER) - glycogenolysis - glycogenesis
Cystic Fibrosis (CFTR gene)
Alanine aminotransferase (ALT) and aspartate aminotransferase (AST); alanine to pyruvate and aspartate to oxaxloacetate

32. What are the priorities in fasting and starvation?
Yes! congenital simply means 'born with'- hereditary is familial
TCA cycle - FA oxidation - acetyl coA production; ox phos
To supply sufficient glucose to the brain and the RBCs and to preserve protein
Alcohol dehyrogenase (alcohol to acetaldehyde); acetaldehyde dehydrogenase (acetaldehyde to acetate); NAD+; B3 (niacin)

33. What is the rate determining enzyme in glycogenolysis?
Decreased substrate for gluconeogenesis (pyruvate to lactate and not oxaloacetate); and pyruvate doing only lactic acidosis and not glycolysis
Glycogen phosphorylase
Glycerol goes to liver for gluconeogenesis; free fatty acids bind to albumin in blood and travel to cells to be used as for energy
Do agarose gel eletrophoresis to separate the different sizes of the PCR products (sizes id the the fragment)

34. What is heteroplasmy?
Presence of normal and mutated mtDNA (like mosaicism except in mit)
paternal
Meconium ileus and failure to thrive; negative serum immunoreactive trypsin
Heteroplasmy

35. What is the rate determining enzyme in HMP shunt?
H1
Excreted from the kidney in the form of urea in hepatocytes
aldose reductase
Glucose 6 phosphate dehydrogenase

36. Achondroplasia is associated with advanced ________ age
Metaphase (all lined up); blood - bone marrow - amniotic fluid - placental tissue; used to diagnose chromosomal abnormalities (deletions - trisomies - sex chromosomal disorders)
Uric acidemia; HGPRT; purine salvage
paternal
Gets converted to pyruvate via the Cori cycle in the liver

37. What is unique about the diarrhea in lactase deficiency?
Bacteria produce lactate acid and gases from lactose and produce acidic diarrhea and bloating
Autosomal Dominant; NF1 and mit. diseases (variable expression) - ?
Very Poor Carb Metabolism (Von Gierke - Type 1 - Pompes - Type II - Cori's - Type III - McArdles disease - Type IV)
Dopa decarboxylase; B6

38. What two anti - neoplastic drugs work by blocking DNA and RNA polymerase?
Dactinomycin - doxorubicin
to ions
Phenylalanine hydroxylase or tetrahydrobiopterin factor
FISH

39. How is entry into cell cycle phases regulated?
By cyclin CDK complexes
10 - 9 - 7 - 2; protein C and S
Cobalamin; cofactor for homocysteine methyltransferase (transfers SAM to THF to give to dUMP) and for methylmalonyl coA mutaseto make succinyl coA
Prolonged deficiency of Vitamin C results in decreased tensile strength of collagen because lack of hydroxylated lysines to crosslink (hydroxylation required vit C); bleeding gums - bone pain - poor wound healing - hemarthroses - glossitis - hemorrha

40. From What aa do NAD+ and NADP+ come from?
Tryptophan
Fructokinase; benign - fructose in the blood and urine
glutton
homocysteine transferase

41. What carries electrons (as energy)?
GTP: gluconeogenesis - OXA to phosphoenolpyruvate
Glycogen synthase
NADH - NADPH - FADH2
1) glycogenolysis (liver only contributes to plasma - muscle keeps it for itself) 2) gluconeogenesis

42. What else decreases production of oxaloacetate in alcoholism other than decreased pyruvate?
Cylindrical made up of a helical array of polymerized alpha and beta tubulin dimers - each dimer has 2 GTP bound
Oxaloacetate gets shifted to malate d/t excess NADH
fructose
teratogenic

43. What is the rate determining enzyme in galactose metabolism?
kidney
GALT
Decrease AFP - increased BCHG - decreased estriol - increased inhibin A
Its When two acrocentric chromosomes combine their long arms (and short arms disappear); When the translocation is functional

44. What disease is associated with chromosome 16?
Uracil; thymine
2 NADH - 2 ATP - 2 pyruvate - 2H+ - 2H20
Cri du Chat
APCKD

45. What generally results after Vitamin B1 deficiency? T/F treat b1 def - with glucose.
ATP depletion and highly aerobic tissues (heart and brain) are affected first; false - glucose infusion worsens
Riboflavin makes up FAD and FMN both used in redox reactions; vegans
Mental retardation - rocker bottom feet - micropthalmia - microcephaly - cleft lip/palate - holoprosoncephaly - polydactyly
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

46. What two def. most commonly cause phenylketonuria?
catecholamine synthesis (dopamine to norepinephrine)
Acetyl CoA carboxylase
Phenylalanine hydroxylase or tetrahydrobiopterin factor
No; increased mutation rate!

47. Loss of heterozygosity is the rule for...
Differentiation of cells - vision in reduced light (retinol makes up rhodopsin cells) - antioxidant
tumor suppressor genes (p53 - Rb - BAX)
Arylsulfatase A; cerebroside sulfate
liver

48. What disease is associated with chromosome 3?
VHL (Von Hippa Lindau)
Arg - lys - his; positive except His has no charge at body pH
Creates nicks in DNA to prevent supercoiling; both!
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

49. What causes an excess of B3? What are the symptoms?
Genotyping for mutant alleles (different sizes)
Niacin treatment for hyperlipidemia; facial flushing
Homocysteine methyltransferase - cystathionine reductase - decreased affinity of cystathionine reductase for pyridoxal phosphate; AR
Consumption of bear liver - and overtreatment isoretinoin; constitutional symptoms and alopecia and skin changes

50. What is the order of cell cycle phases? What happens in each phase?
Orotic aciduria megaloblastic anemia can not be corrected with folic acid or vit B12
NADPH- reductive biosynthesis!
Oral uridine administration; provides nucleosides and provides feedback inhibition
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)