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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 a robertsonian translocation? What does it mean if its balanced?
30S ribosomal RNA (small one)
Avidin in raw egg whites prevents their reabsorption; also antibiotic use; dermatitis - alopecia - glossitis - lactic acidosis - enteritis
Cardiomegaly - cyanosis - tachycardia - dyspnea - vomitting; 2-3 months
Its When two acrocentric chromosomes combine their long arms (and short arms disappear); When the translocation is functional

2. What are the mucopolysaccharidoses? Which one is XR? AR? What accumulates in them?
Damaged 'ubiquinated' cytokeratin intermediate filaments build up in hepatocytes - damaged neurofilaments in idiopathic parkinsons disease build up to form Lewy bodies - Neurofibrillary tangles are build up of neurofilaments in alzheimers
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
Hydroxurea; deoxygenates ribose
Hurlers Syndromes;Iduronate sulfatase deficiency; alpha L iduronidase def; heparan sulfate - dermatan sulfate

3. 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
Multiple bone fractures - blue sclerae - hearing loss (abdnormal middle ear bones!) - dental imperfections - retarded wound healing
When water is pulled in; polyols (sugar alcohols); cataracts - retinopathy - peripheral neuropathy all seen in chronic hyperglycemia in diabetes
Using serine (which turns into glycine after reaction)

4. What is the fate of glucose from glycogenolysis?
Either salvage by HGPRT and PRPP or conversion to xanthine and then uric acid by xanthine oxidase
Vomiting - rice water stools - garlic breath
Glucosekinase to glucose 6 P
Hemorrhage with increased PT and PTT time but normal bleeding time

5. What are five clinical features seen in Osteogenesis Imperfecta?
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
Metaphase (all lined up); blood - bone marrow - amniotic fluid - placental tissue; used to diagnose chromosomal abnormalities (deletions - trisomies - sex chromosomal disorders)
Multiple bone fractures - blue sclerae - hearing loss (abdnormal middle ear bones!) - dental imperfections - retarded wound healing
Insertion or deletion of a number of nucleotides indivisible by 3; tay sachs - duchennes MD

6. What is the difference between wet and dry beriberi?
DHF
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
Distinctive elfin faces - mental retardation - hypercalcemia d/t sensitivity to vit D - well developed verbal skills and extreme friendliness - CV problems
= q

7. What are the clinical findings in edwards syndrome?
Mental retardation - rocker bottom feet - micrognathia (small jaw) - low set ears - clenched hands with polydactyly - congenital heart disease
S adenosyl homocysteine; homocysteine transferase; B12 and THF; M- THF
Glutamate
Release alk phos

8. What type of aa are histones made of? are they acidic or basic?
Increased orotic acid in urine - megaloblastic anemia - +/- hyperammonemia
Lysine and arginine; basic
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
Aldolase B; Fructose 1 phosphate accumulates Which uses up Phosphate and the result is inhibition of gluconeogenesis without phosphate; hypoglycemia - jaundice - cirrhosis - vomitting

9. What is the source of ATP When you are fasting (in between meals)?
Increased blood levels of ammonia - decreased BUN
Hepatic glycogenolysis (mostly) - also hepatic gluconeogenesis - adipose release of FFA (minor)
Liver; P450 hydroxylation
To amplify a specific desired fragment of DNA

10. 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)?
Cell signaling defect in FGF receptor resulting in dwarfism and short limbs with normal head and trunk; AD
Collagen; fibroblasts; 4; ECM
Alpha amanitin; mushroom caps
Electron transport inhibitors - ATPase inhibitors - Uncoupling agents (increase permeability of the membrane - loss of hydrogen gradient)

11. What causes scurvy? What are the symptoms of scurvy?
Abetalipoproteinemia and increased G3P production from all the carbs!
Excreted from the kidney in the form of urea in hepatocytes
Vit C deficiency; glossitis - anemia - weakened vessels - hemorrahges - hemarthroses - swollen gums - bruises -
Von gierkes

12. Vit A is ________ - causes cardiac problems and cleft palate in newborns
Glutamate
Increased CPK and muscle biopsy
10 - 9 - 7 - 2; protein C and S
teratogenic

13. What type of disease is MERRF? What is the clinical presentation?
Myoclonus epilepsy
All and Alzheimers
HMP shunt (to NADPH) and glycogenolysis (as glucose 1 phosphate)
By giving high amounts of pyridoxal phosphate in the diet

14. What explains the fatty change in kwashiokor?
Cobalamin; cofactor for homocysteine methyltransferase (transfers SAM to THF to give to dUMP) and for methylmalonyl coA mutaseto make succinyl coA
Damaged 'ubiquinated' cytokeratin intermediate filaments build up in hepatocytes - damaged neurofilaments in idiopathic parkinsons disease build up to form Lewy bodies - Neurofibrillary tangles are build up of neurofilaments in alzheimers
Beta glucocerebrosidase; glucocerebroside
Abetalipoproteinemia and increased G3P production from all the carbs!

15. What is the pathogenesis of skurvy? name some clinical symptoms.
It contains the peptidyltransferase activity (for elongation of protein
hexokinase (glucokinase only found in liver and beta cells of pancreas)
Lactose (glucose and galactose)
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

16. What does ultrasound show in Downs?
Increased nuchal translucency
Tyrosine
Highly condensed forms of DNA; mitosis; neoplasia - highly dividing and thus hyperchromatic
Tyrosine

17. How is entry into cell cycle phases regulated?
Tyrosine; tryptophan
To transfer methyls; a methionine and an ATP; norepinephrine plus a methyl makes epinephrine
Abnormal glycogen metabolism results in accumulation of glycogen in cells
By cyclin CDK complexes

18. Daughters of Xlinked recessive diseased fathers are...
all carriers
hexokinase (glucokinase only found in liver and beta cells of pancreas)
Acetyl coA; gluconeogenesis
3 NADH - 1 FADH2 - 2 CO2 - 1 GTP = 12 ATP/acetyl CoA

19. 'Initiation' of protein tranlsation is activated by ATP hydrolysis. T/F?
When the amino acid is covalently binded to its corresponding tRNA; Aminoacyl tRNA synthetase; ATP; translation
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
False; GTP hydrolysis
usually present late

20. What causes cataracts in a galactose metabolism deficiency?
Aldose reductase in the lens converts built up galactose to galactitol Which is osmotically active
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
Silent - misssense - nonsense
Tryptophan; niacin

21. How do you treat niacin deficiency do to corn based diet?
high Vmax
Adrenal medulla
By addition of mannose 6 phosphate to proteins in the Golgi
Lime and CaCO3 to displace the niacin

22. What are the resp. clinical symptoms of CF?
Double stranded RNA is created that is complementary to the mRNA of interest and When inserted into cells the strands separate and attach to the mRNA preventing it from being translated
Lactating mammary glands - adrenal cortex (sites of fatty acid or steroid synthesis) - RBCS (for glutathione reduction for FR injury)
Hemolytic anemia; jaundice in newborn if mom gets too much vit K
Thick mucus plugs lungs recurrent pulmonary infections (pseudomonas and aureus) - chronic bronchitis - bronchiectasis (which can lead to cor pulmonale!); biofilms form on resp tract

23. Which amino acid is the most basic?
COP II
Vit K; neonates have sterile intestine and can not synthesize vit K
glucagon - insulin and epinephrine
Arg

24. Where does synthesis of longer chain fatty acids occur? What does it require?
Galactokinase; galactose appears in blood and urine - sometimes infantile cataracts may present as failure to track objects or lack of a social smile
MT!
BM; laminins; most flexible; different aa composition
SER and mitochondria; NADPH

25. ____ soluble vitamins are more likely to result in toxicity
CNS defects - mental retardation - death - feeding difficulties - vomiting - hypoglycemia; thiamine
fat
S. aureus - Aspergillus
Phosphorylation of glucose

26. What is given to prevent side effects When methotrexate is given? why does this work?
Immediately after replication - DNA is unmethylated and so it is a signal to be double checked; HNPCC; heterochromatin
Dynein - retrograde - kinesin - anterograde
Hexosaminidase A; GM2 ganglioside
Folinic acid; does not require DHF

27. _____________ prevents fluidity of plasma membrane
Chorea and spastic paresis
high cholesterol
Schilling test
Do an RT PCR (reverse transcriptase)- RT the RNA first - then PCR the DNA

28. IMP is degraded to inosine and then to hypoxanthine. What two options are next? by What enzymes?
Which dimerizes and activates a protein phosphatase Which de phosphorylates glycogen phosphorylase
Either salvage by HGPRT and PRPP or conversion to xanthine and then uric acid by xanthine oxidase
NADPH- reductive biosynthesis!
Phosphofructokinase 1; fructose 1 phosphate to fructose 1 -6 bisphosphate; enhancers: AMP - fructose 2 -6 Bisphosphate; downregulators: citrate - ATP

29. All glutamate goes to _______ d ammonia is used as a buffer in urine
kidney
Severe fasting hypoglycemia - very high levels glycogen in liver - high blood lactate - hepatomegaly
Autosomal Dominant; NF1 and mit. diseases (variable expression) - ?
Abs against type IV collagen; pulmonary and glomerular capillaries

30. What does fructose 2 -6 bisphosphate regulate?
Chloramphenicol
Positive on phosphofructokinase 1 and negative on fructose 1 -6 bisphosphatase
3; 5; mit matrix; alpha ketoglutarate dehydrogenase complex
7 dehydrocholesterol

31. Give an example of codominance.
Carbamoyl phosphate
Blood group types
Phosphorylation of glucose to yield glucose 6 phosphate; ATP; first step of glycolysis
reabsorbs

32. What is the treatment of hyperammonia?
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)
Riboflavin makes up FAD and FMN both used in redox reactions; vegans
HnRNA; mRNA
be anorexic

33. What can cause Biotin deficiency? What are the symptoms?
Avidin in raw egg whites prevents their reabsorption; also antibiotic use; dermatitis - alopecia - glossitis - lactic acidosis - enteritis
30S ribosomal RNA (small one)
Gets converted to pyruvate via the Cori cycle in the liver
Tyrosine

34. Where is type 1 collagen found?
Bone - Skin - Tendon - dentin - fascia - cornea - late wound repair
P53 and Rb; p53 activates BAX Which inhibits BCL2 When there is DNA damage - Rb phosphorylation allows progression into S phase
HVA
It inhibits conversion of DHF to THF by blocking dihyrdofolate reductase

35. What liver enzymes are increased in alcoholism? why?
NADH - NADPH - FADH2
AST (mit damage) - and GGT (d/t SER hyperplasia d/t induction of p450 generation)
Fructose 1 -6 bisphosphatase
ATP and biotin

36. What is a clinical example of a missense mutation?
Bendy bones seen in children due to decreased Vit D; craniotabes - skeletal deformities - growth retardation - rosary
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
Double stranded RNA is created that is complementary to the mRNA of interest and When inserted into cells the strands separate and attach to the mRNA preventing it from being translated
Sickle cell disease - conversion of hydroophillic glutamic acid to hyrdophobic valine

37. What type of polymerase is primase?
Later
Lupus
all carriers
DNA dependent RNA polymerase

38. why does branching occur in glycogenesis?
only yield acetyl coA
Branching increases the rate of synthesis and breakdown of glycogen by having more sites to add on to and break off
Pataus
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
Aldolase B
fructose
Respiratory burst - P450 - and glutathione reductase
Vit K; neonates have sterile intestine and can not synthesize vit K

40. What carries electrons (as energy)?
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
Cystathionine (which then goes to cysteine) via cystathionine reductase; B6
Homocysteine; homocysteine methyltransferase with B12 and tetrahydrofolate
NADH - NADPH - FADH2

41. What is the pyruvate dehydrogenase complex used for? What is produced in that reaction?
Phenylalanine build up
N linked oligosaccharides
MT!
Conversion of pyruvate to Acetyl CoA; acetyl coA - CO2 - and NADH (one of each per pyruvate)

42. How do WBCs of CGD patients do some damage?
They use the H2O2 produced by some bacteria to produce bleach with myeloperoxidase Which they still have
Cri du Chat
S adenosyl homocysteine; homocysteine transferase; B12 and THF; M- THF
Carbomyl phosphate synthetase I

43. What aa (other than glutamate) is a precursor for urea?
25 OH D3; 1 -25 (OH)2 D3 (Calcitriol)
Helps make Vit K and antioxidant
Nitric Oxide - creatinine - Urea
Arginine

44. Where is vit C found?
dimers
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
Fruits and vegetables
Enriched whole grain cereals and other things; thiamine; malnutrition (nonenriched white processed bread and rice) and alcohol second to malnutrition

45. What toxins prevent elongation in translation in humans (by binding 40s)?
I; nucleolus
Positive on phosphofructokinase 1 and negative on fructose 1 -6 bisphosphatase
NF2 - DiGeorge/Velocardiofacial syndromes
Shiga toxin and ricin (protein in castor beans)

46. What dose a deficiency in Vit B12 cause? What causes it?
is asymmetric
Complex I - Complex II (lower energy)
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
Tyrosine

47. How is the nitrogen of amino acids transferred to the liver for the urea cycle? explain this process.
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
As alanine; amino acids donate their amino group to alpha ketoglutarate making it glutamate and the aa then becomes an alpha ketoacid (joins TCA cycle); glutamate then converts pyruvate to alanine by donating its amino group to pyruvate (with ALT); a
osteosarcomas (Rb mutation is in all cells)
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

48. is CPSII or CPSI involved in pyrimidine synthesis?
CPSII
LDL; LDL oxidize atherosclerosis
Cytoplasmic side; active
Fructose bisphosphatase 2 and Phosphofructokinase 2; phosphorylation by protein kinase A; inuslin/glucagon ratio

49. What does deficiency of Vit K cause?
Hemorrhage with increased PT and PTT time but normal bleeding time
Degeneration of retinal ganglion cells and axons; acute loss of central vision; mit. inheritance
Oxygen!
No; increased mutation rate!

50. AR disease is usually...
Mental retardation - excessive laughter - seizures - ataxia - 'happy puppet'
TRNA wobble
severe
Cylindrical made up of a helical array of polymerized alpha and beta tubulin dimers - each dimer has 2 GTP bound

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USMLE Step 1 Biochemistry

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