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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.
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1. is CPSII or CPSI involved in pyrimidine synthesis?
A ketone
the RER
CPSII
Lactose (glucose and galactose)

2. Where does a majority of galactose in our diet come from?
pleiotropic
B12 and folate; B12; washed out
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
Lactose (glucose and galactose)

3. Homocysteine gets turned into methionine by...
Meiotic nondisjunction in mothers gametes during the first meiosis
homocysteine transferase
Mcardles disease
Cri du Chat

4. What metabolic reactions occur in the cytoplasm?
By activating adenylyl cylase Which increases cAMP Which increases levels of protein kinase A Which activates glycogen phosphorylase kinase; epinephrine
Renal failure - decreased sun - fat malabsorption - chronic liver disease - p450 induction - hypoparathyroidism (activates 1 alpha hyrdoxylase)
ATP and dATP
Glycolysis - fatty acid synthesis - HMP shunt - protein synthesis (RER) - steroid synthesis (SER) - glycogenolysis - glycogenesis

5. AD disease...
Beckers is less severe (instead of deletion is either defect or deficiency) with later presentation
usually present late
III
Trinucleotide expansion diseases

6. What does a def in folic acid cause? is it more or less common than cobalamin def? How do you differentiate it with cobalamin def?
Fat soluble; either endogenous production or exogenous; D2 ergocalciferol is ingested from plants and D3 cholecalciferol is formed in sun exposed skin (and can be consumed in fortified milk)
O2 consumption (last electron acceptor) gets consumed also
NF1
Macrocytic megaloblastic anemia; it is the most common vit def in the US; no neuro symptoms and no methylmalonyl coA in the urine

7. What are the clinical symptoms associated with fragile X syndrome?
Decreased phenylalanine - increase tyrosine
NAD+; through ETC you produce NAD+; through production of Lactate from pyruvate you create lactate
Macroorchitism - big ears - big jaw - mental retardation (2nd mcc!) - mitral valve prolapse - long face
Malabsorption problems (CF and sprue) and mineral oil intake

8. What is the most common cause of Osteogenesis Imperfecta? What is the inheritance pattern?
severe
Type I collagen; AD
GTP: gluconeogenesis - OXA to phosphoenolpyruvate
is asymmetric

9. What is the degradation product of dopamine?
Assist in spontaneous refolding of proteins
Marfans - homocystinuria - MEN 2B
usually present late
HVA

10. What is most often mutated in xeroderma pigmentosum? What is the inheritance pattern? What are the clinical symptoms?
UV specific endonucleases for excision repair; AR; dry skin - photosensitivity - poikiloderma - susceptibility to skin cancers
Pernicious anemia; older - mentally slow european woman who is anemic and icteric with a smooth shiny tongue and has broad based shuffling gait
Met - arg - val - his
To amplify a specific desired fragment of DNA

11. What causes an excess of B3? What are the symptoms?
Robertsonian translocations
No protein!
Niacin treatment for hyperlipidemia; facial flushing
Phenylethanolamine N- methyl transferase; SAM

12. What is the difference between methotrexate and trimethroprim?
No
Trimethroprim inhibits bacterial dihyrofolate reductase
calorie deficient
Less; more

13. What does topoisomerase do? I or II eukaryotic?
Creates nicks in DNA to prevent supercoiling; both!
Limit dextrans
Cysteine; tyrosine
Carbomyl phosphate synthetase I turns N acetyl glutamate into carbomyl phosphate and then ornithine transcarbomyalse turns carbomyl phosphate into citrulline (using ornithine)

14. Findings for Cori's disease? deficient enzyme?
Milder form of type 1 with normal blood lactate levels; debranching enzyme (alpha -1 -6 glucosidase)
Galactose
Schilling test
HVA

15. What vesicular trafficking protein takes proteins from the Golgi to lysosomes? in between the golgi? from the plasma membrane to endosomes (receptor mediated endocytosis)?
Tryptophan; niacin
activation of PFK1
All clathrin
Familial Adenomatous Polyposis (APC gene)

16. What converts fructose 2 -6 phosphate back to fructose 2 phosphate?
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
Cysteine; tyrosine
Fructose bisphosphatase 2
Catalyze gamma carboxylation of glutamic acid residues on coag factors so they can bind to Ca complex

17. Other than being hereditary - What are two other ways you can have a lack of brush border enzymes?
Succinate hydrogenase; matrix
Cafe au lait spots - optic pathway gliomas - lisch nodules - scoliosis - neural tumors
Following gastroenteritis and kwashiokor (protein deficiency)
AR

18. What aa is the ultimate precursor of catecholamine synthesis?
AD; disorder of blood vessels that results in telangietasias - skin discolorations - and AVMs (arteriovenous malformations)
ATP depletion and highly aerobic tissues (heart and brain) are affected first; false - glucose infusion worsens
can be d/t lyonization
Phenylalanine

19. What is the effect of the Cardiac glycosides?
Defective neutral amino acid transporter on renal and intestinal cells; AR
Bone - Skin - Tendon - dentin - fascia - cornea - late wound repair
Indirect inhibtion of the Na+/Ca+2 exchange resulting in increased intracellular calcium Which increases cardiac contractility
No protein!

20. In What cells does the Respiratory burst occur? Where is NADPH located in the cell? What is this reaction important for?
Decreased substrate for gluconeogenesis (pyruvate to lactate and not oxaloacetate); and pyruvate doing only lactic acidosis and not glycolysis
Progressive neurodegeneration (spasticity d/t UMN disease) - hepatosplenomegaly - cherry red spot on macula - foam cells
Neutrophils and monocytes; membrane bound; immune response by generating Reactive oxygen species
fastest sugar

21. What explains the excess phenylketones in the urine in phenylketonuria?
They use the H2O2 produced by some bacteria to produce bleach with myeloperoxidase Which they still have
Phenylalanine build up
Excreted from the kidney in the form of urea in hepatocytes
Glycosylases (only remove the base - cut the bond between ribose and base)

22. Fructose 2 -6 bisphophate is involved in...
is active
D2 (ergocalciferol) - D3 (cholecalciferol) - D3 (cholecalciferol)
allosteric regulation
Following gastroenteritis and kwashiokor (protein deficiency)

23. What is a telangiectasia? Where can they be found?and What is a common result of them?
Dilated capillaries and veins; skin - mucous membranes - GI - resp - urinary tracts; rupture and can cause epistaxis - hemorrhage - GI bleeding
Cytosine to uracil
dimers
Alpha 1 -4 linkages; alpha 1 -6 linkages

24. What is the major cause of SCID? What does SCID stand for?
Adenosine deaminase deficiency; Severe combined immunodeficiency disease
Karyotyping
Amniocentesis karyotyping
Release alk phos

25. The mode of inheritance for both NF disease is...
Vitamin C; facilitates iron absorption by making sure its in its reduced state (Fe2+) - hydroxylation for lysine and proline Which is necessary for crosslinking; necessary for dopamine Beta hydroxylase for conversion of dopamine to NE; keeps FH4 in i
Both AD
5' end; 3' end
ATP depletion and highly aerobic tissues (heart and brain) are affected first; false - glucose infusion worsens

26. How are senescent cells different from some neoplastic and germ cells? What does this substance do?

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27. What glucose is stored as
glycogen
Bone - Skin - Tendon - dentin - fascia - cornea - late wound repair
Very Poor Carb Metabolism (Von Gierke - Type 1 - Pompes - Type II - Cori's - Type III - McArdles disease - Type IV)
Fructose bisphosphatase 2 and Phosphofructokinase 2; phosphorylation by protein kinase A; inuslin/glucagon ratio

28. Heterozygous females in X linked recessive disease...
Fructose gets phosphorylated into fructose 1- P by fructokinase and then fructose 1P gets metabolized into DHAP or glyceraldehyde by aldolase B
NADPH oxidase; CGD
Chromosomal imbalance (downs for ex)
can be d/t lyonization

29. In protein translation - What is the 'A' site for? 'P' site? 'E' site? What is the exception to this?

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30. After how long of starvation do glycogen reserves deplete? after this occurs - how are blood glucose levels maintained?
FA coA synthetase combines coA with FA to make acyl coA
1 day; adipose release of FFA - hepatic gluconeogenesis from peripheral tissue lactate and alanine and from adipose tissue glycerol and propionyl coA
Aldose reductase in the lens converts built up galactose to galactitol Which is osmotically active
A lysosomal storage disease; mannose 6 phosphate can not be added to lysosomal proteins

31. Name a few intermediate filament defects. hints: alcohol - 2 neuro dx

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32. What step in pyrimidine synthesis requires aspartate?
Silent - misssense - nonsense
Aldolase B
Carbamoyl phosphate to orotic acid
Trimethroprim inhibits bacterial dihyrofolate reductase

33. Which metabolic reactions require both the mit and cytosol?
Hydroxurea; deoxygenates ribose
Heme synthesis - urea cycle - gluconeogenesis
To supply sufficient glucose to the brain and the RBCs and to preserve protein
Tremor (asterixis) - slurring of speech - decrease temperature - somnolence - vomitting - cerebral edema - blurring of vision - neural focal deficits

34. What enzyme converts norepinephrine to epinephrine? using What cofactor?
Osteoblasts; de phosphorylates pyrophosphate Which inhibits bone mineralization; bone mineralization
Renal failure - decreased sun - fat malabsorption - chronic liver disease - p450 induction - hypoparathyroidism (activates 1 alpha hyrdoxylase)
Phenylethanolamine N- methyl transferase; SAM
CNS defects - mental retardation - death - feeding difficulties - vomiting - hypoglycemia; thiamine

35. What two anti - neoplastic drugs work by blocking DNA and RNA polymerase?
Dactinomycin - doxorubicin
Mit encephalopathy - lactic acidosis - stroke like episodes
teratogenic
Neural tissue and adrenal medulla

36. What are the lab findings see in hyperammonemia?
hexokinase (glucokinase only found in liver and beta cells of pancreas)
Increased blood levels of ammonia - decreased BUN
Debranching enzyme; alpha 1 - 4 glucosidase; no!; Pompes disease
Glycine; heme

37. What is the signal for adenylation? What enzyme does? does it require a template?
No; increased mutation rate!
AAUAAA; Poly A polymerase; no
plasma membrane
fatty acid degradation

38. Which lysosomal storage diseases have enzymes that are def in cells that are neuronal mostly?
Do an RT PCR (reverse transcriptase)- RT the RNA first - then PCR the DNA
SnRNPs; RNA polymerase III
Neimann pick and tay sachs
Vitamin C; facilitates iron absorption by making sure its in its reduced state (Fe2+) - hydroxylation for lysine and proline Which is necessary for crosslinking; necessary for dopamine Beta hydroxylase for conversion of dopamine to NE; keeps FH4 in i

39. Do RNA polymerases have proofreading function? What does this mean for HIV?
No; increased mutation rate!
HMP shunt (to NADPH) and glycogenolysis (as glucose 1 phosphate)
Decrease AFP - increased BCHG - decreased estriol - increased inhibin A
TPP (from B1- pyrophosphate) - FAD (B2) - NADH2 (B3) - CoA (B5 - panothenate) - Lipoic Acid

40. 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)?
UV specific endonucleases for excision repair; AR; dry skin - photosensitivity - poikiloderma - susceptibility to skin cancers
Collagen; fibroblasts; 4; ECM
HMG CoA reductase
Urea cycle enzyme deficiency (mc: ornithine transcarbamoylase); liver disease - alcoholic cirrhosis and reye's

41. What are the findings in alkaptonuria? why do they occur?
Lesch Nyhan syndrome; X linked recessive
Histidine; B6
Pyridoxine; converted to pyridoxal phsophate used in transamination reaction - decarboxylations - heme synthesis - niacin synthesis from trp - glycogen phosphorylase - cystathionine synthesis - NT synthesis
Dark connective tissue - brown pigmented sclera - debilitating athralgias; homogentistic acid builds up and can be toxic to cartilage

42. What converts dopa into melanin?
Acetyl coA; gluconeogenesis
Tyrosinase
Phosphofructokinase 1
It inhibits conversion of DHF to THF by blocking dihyrdofolate reductase

43. Which amino acids are acidic? are they negatively or positively charged at body pH?
Consumption of bear liver - and overtreatment isoretinoin; constitutional symptoms and alopecia and skin changes
Delayed wound healing - hypogonadism - decreased adult hair - dysgeusia - anosmia - and alcoholic cirrhosis - perioral rash
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
Asp and glu; negatively

44. What is the result of arginase deficiency?
Mebendazole and thiabendazole
Myoclonus epilepsy
only yield acetyl coA
Chorea and spastic paresis

45. Glucokinase is a...
2 oxidative and nonoxidative; cytoplasm (both); none (none produced either - you are simply switching carriers of energy- from glucose 6 phosphate to NADPH)
Neurologic defects - myopathy - lactic acidosis; giving ONLY ketogenic nutrient high fat content and aa (lysine and leucine)
Citrate is positive on it - AMP and fructose 2 -6 bisphosphate is negative on it; gluconeogenesis
glutton

46. why does branching occur in glycogenesis?
NF1
Branching increases the rate of synthesis and breakdown of glycogen by having more sites to add on to and break off
Pigmented iris hamartomas
A 16C FA! the product in FA synthesis; saturated; no double bonds

47. What is the pathogenesis of fragile x syndrome?
A trinucleotide repeat disorder results in faulty methylation and thus expression of the FMR1 gene on the X chromosome (x linked recessive mode of inheritance)
(cofactor for 100+ enzymes!) needed for zinc fingers transcription motif - collagen synthesis - alcohol metabolism
UV nonionizing radiation causing thymidine dimers
Silent - misssense - nonsense

48. From What aa do NAD+ and NADP+ come from?
cell cycle events
Glutamate and aspartate; CO2 provides an ketone
Tryptophan
A trinucleotide repeat disorder results in faulty methylation and thus expression of the FMR1 gene on the X chromosome (x linked recessive mode of inheritance)

49. In What genetic code is uracil found in? in replace of what?
Neurons and RBCs
Pyruvate! (not lactate - that is generate just in order to produce more NAD+)
Uracil; thymine
Gauchers; hepatosplenomegaly - aseptic necrosis of the femur - bone crises - Gauchers cells - crumpled tissue paper looking (fibrillar appearing macrophages in liver - spleen - b.m.)

50. What creates a frameshift mutation? What are some clinical examples?
Niacin treatment for hyperlipidemia; facial flushing
Insertion or deletion of a number of nucleotides indivisible by 3; tay sachs - duchennes MD
Glycosylases (only remove the base - cut the bond between ribose and base)
Inability to transport LCFA into mitochondria results in their build up and causes weakness - hypotonia And HYPOKETOTIC HYPOGLYCEMIA

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USMLE Step 1 Biochemistry

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