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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 are the symptoms of cri du chat syndrome? What is the pathogenesis?
Decrease AFP - increased BCHG - decreased estriol - increased inhibin A
Cylindrical made up of a helical array of polymerized alpha and beta tubulin dimers - each dimer has 2 GTP bound
Microcephaly - mental retardation - high pitched mewing/crying - epicanthal folds - cardiac abnormalities; microdeletion on short arm of chrom 5
Cafe au lait spots - optic pathway gliomas - lisch nodules - scoliosis - neural tumors

2. What does a pyridoxine deficiency look like? What causes it?
Methionine; f Met (formyl methyl methionine)
A lysosomal storage disease; mannose 6 phosphate can not be added to lysosomal proteins
Dynein - retrograde - kinesin - anterograde
Convulsions - hyperirritiability - peripheral neuropathy - sideroblastic anemias; alcoholics and INH and fortified goats milk consumption

3. Which is the rate limiting step of the urea cycle?
Tyrosine
Decreased substrate for gluconeogenesis (pyruvate to lactate and not oxaloacetate); and pyruvate doing only lactic acidosis and not glycolysis
Carbomyl phoshpate synthetase I
fat

4. What are chromosomes? during What part of the cell cycle are they found? in What pathologic state would you see a lot of chromosomes?
Iatrogenic - granulomatous disease (sarcoidosis d/t epitheloid macrophage release of calcitriol); hypercalcemia - hypercalciuria (renal calculi!) - loss of appetite - stupor
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
Vincrstine (vinca alkaloids) and paclitaxel
Highly condensed forms of DNA; mitosis; neoplasia - highly dividing and thus hyperchromatic

5. What are the clinical symptoms associated with fragile X syndrome?
Oral uridine administration; provides nucleosides and provides feedback inhibition
Macroorchitism - big ears - big jaw - mental retardation (2nd mcc!) - mitral valve prolapse - long face
anabolic processes
Cortisol; inhibits insulin action

6. Phosphorylated glycogen phosphorylase...
= q
XR - iduronate sulfatase
ATP is an allosteric inhibitor on both phosphofructokinase and pyruvate kinase and pyruvate hydrogenase; and fructose 2 -6 BP Which is an allosteric enhancer on phosphofructokinase is an allosteric downregulator on fructose 1 -6 bisphophatase
is active

7. What is lebers hereditary optic neuropathy? What is the main clinical symptom? and What is its mode of inheritance?
Degeneration of retinal ganglion cells and axons; acute loss of central vision; mit. inheritance
Pyruvate! (not lactate - that is generate just in order to produce more NAD+)
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
Block ATP synthase resulting in no ATP production and an increased proton gradient

8. What are the functions of zinc?
Phosphoenolpyruvate; pyruvate kinase; fructose 1 - 6 BP; ATP - alanine
is active
(cofactor for 100+ enzymes!) needed for zinc fingers transcription motif - collagen synthesis - alcohol metabolism
Macrocytic megaloblastic anemia; it is the most common vit def in the US; no neuro symptoms and no methylmalonyl coA in the urine

9. How do you differentiate causes of megaloblastic anemia?
Kidney; 1 alpha hydroxylase; Parathyroid hormone
Orotic aciduria megaloblastic anemia can not be corrected with folic acid or vit B12
Chorea and spastic paresis
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

10. What enzyme is deficient in classic galactosemia? symptoms? treatment?
Ceramide trihexoside
Arg
can be d/t lyonization
Galactose 1 phosphate uridyltransferase (GALT) results in an accumulation of toxic substances; failure to thrive - jaundice - hepatomegaly - infantile cataracts - mental retardation; exclude galactose and lactose from diet

11. What builds up in the blood of patients with maple syrup urine disease? urine/poo? What does this cause?
A topoisomerase II inhibitor; because cancer cells use II more so than healthy cells
Coarse facial features - psychomotor retardation - clouded corneas - restricted joint movement - high plasma levels of lysosomal enzymes
Glycolysis - fatty acid synthesis - HMP shunt - protein synthesis (RER) - steroid synthesis (SER) - glycogenolysis - glycogenesis
Alpha ketoacids in the blood; alpha keto acids in the urine/poo smells of maple syrup/burnt sugar

12. In the genetic code - What does unambiguous mean?
Niemann pick and Tay Sachs; hepatosplenomegaly or not
Cardiomegaly and systemic findings leading to early death; lysosomal alpha -1 -4- glucosidase (acid maltase)
Each codon specifies only one amino acid
GTP: gluconeogenesis - OXA to phosphoenolpyruvate

13. The ________ is 50% cholesterol - 50% phoshpolipids
Nacetyl cysteine; breaks disulfide bridges of mucus plugs
Robertsonian translocations
plasma membrane
Tyrosine; phenylaline

14. Loss of heterozygosity is the rule for...
tumor suppressor genes (p53 - Rb - BAX)
Mucus secreting goblet cells - ab secreting plasma cells - nt secreting neurons
Down's Syndrome
Coarse facial features - psychomotor retardation - clouded corneas - restricted joint movement - high plasma levels of lysosomal enzymes

15. What is the function of mitDNA?
Encode enzymes for oxphos mit reactions
Avidin in raw egg whites prevents their reabsorption; also antibiotic use; dermatitis - alopecia - glossitis - lactic acidosis - enteritis
Dermatitis - glossitis - diarrhea
CCA and chemically modified bases Where amino acid is covalently bonded

16. What is the metabolism of galactose like?
Lack of proper dietary therapy during pregnancy resulting in infant's microcephaly - mental retardation - growth retardation and congenital heart defects
Silent - misssense - nonsense
Tryptophan; melatonin
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

17. why does a deficiency in ornithine transcarbamoylase (urea cycle enzyme) result in an increased production of orotic acid - the pyrimidine precursor?
Pernicious anemia; older - mentally slow european woman who is anemic and icteric with a smooth shiny tongue and has broad based shuffling gait
Endonucleases - exonucleases - DNA polymerase - DNA ligase
Because carbamaoyl phosphate is involved in both urea cycle and pyrimidine synthesis
A cytoskeletal protein - apart of flagella - cilia - mitotic spindles and transport (in neurons for example)

18. How do you treat homocystinuria due to decreased affinity of cystathionine reductase to B6?
A cytoskeletal protein - apart of flagella - cilia - mitotic spindles and transport (in neurons for example)
Met - arg - val - his
Carbomyl phosphate synthetase II
By giving high amounts of pyridoxal phosphate in the diet

19. What aa does AUG mRNA code for in eukaryotes? prokaryotes?
Chediak Higashi Syndrome; decreased phagocytosis results in recurrent pyogenic infections - partial albinism - and peripheral neuropathy
Methionine; f Met (formyl methyl methionine)
Decreased substrate for gluconeogenesis (pyruvate to lactate and not oxaloacetate); and pyruvate doing only lactic acidosis and not glycolysis
By activating adenylyl cylase Which increases cAMP Which increases levels of protein kinase A Which activates glycogen phosphorylase kinase; epinephrine

20. What does glycogen phosphorylase in glycogenolysis create?
N5N10 methylene THF
Vit C
low Vmax
Limit dextrans

21. After the PCR reaction - How do you actually visualize the DNA?
Depression - progressive dementia - choreiform movements; between the ages of 20 and 50
Imprinting Dx's: prader Willi and Angelmann
Blood group types
Do agarose gel eletrophoresis to separate the different sizes of the PCR products (sizes id the the fragment)

22. What is the rate determining enzyme in galactose metabolism?
Distinctive elfin faces - mental retardation - hypercalcemia d/t sensitivity to vit D - well developed verbal skills and extreme friendliness - CV problems
Consumption of bear liver - and overtreatment isoretinoin; constitutional symptoms and alopecia and skin changes
Cylindrical made up of a helical array of polymerized alpha and beta tubulin dimers - each dimer has 2 GTP bound
GALT

23. What lysosomal storage diseases are seen more in Ashkenazi Jew?
It inhibits conversion of DHF to THF by blocking dihyrdofolate reductase
Ortinithine transcarbamoylase - orotic acid phosphoribosyltransferase - and orotidine 5 phosphate decarboxylase; urea cycle; last two are involved in conversion of orotic acid to UMP (adding PRPP to orotic acid)
Renal calculi of uric acid (replaces uric acid in excretion)
Tay Sachs - Niemann Picks - Gauchers

24. What amino acids are required in purine synthesis? Which of these does pyrimidine synthesis require also?
Less; more
Because carbamaoyl phosphate is involved in both urea cycle and pyrimidine synthesis
Glutamine - aspartate - glycine; aspartate
Alcohol dehyrogenase (alcohol to acetaldehyde); acetaldehyde dehydrogenase (acetaldehyde to acetate); NAD+; B3 (niacin)

25. What is the difference between methotrexate and trimethroprim?
Trimethroprim inhibits bacterial dihyrofolate reductase
Acetone
Alpha amanitin; mushroom caps
Mcardles disease

26. What is one steroid that increases gluconeogenesis? What else does cortisol do that yields hyperglycemia?
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
Cortisol; inhibits insulin action
tubulin dimer
Make it stiffer/more tensile; continues as we get older so gets less and less elastic

27. After glucose 6 P is converted to glucose 1 P - What is the next step in glycogenesis? then what?
Creates nicks in DNA to prevent supercoiling; both!
normal lactate levels
Conversion to UDP glucose via UDP glucose pyrophosphorylase; glycogen synthase (rate limiting step )
Acidic

28. why is there decreased immunity in kwashiokor and marasmus?
When water is pulled in; polyols (sugar alcohols); cataracts - retinopathy - peripheral neuropathy all seen in chronic hyperglycemia in diabetes
Preprocollagen - procollagen - tropocollagen - collagen
No protein!
1 gene has greater than 1 phenotypic effect; locus heterogeneity (many different mutations can cause the same phenotype); PKU; albinism - deafness - Marfanoid habitus (Marfans - MEN 2B - homocystinuria)

29. What causes liver damage in kwashiokor?
both liver and muscle (unlike glucagon)
ATP and dATP
kidney
Increased TG synthesis from increased Glycerol 3 P and abetalipoproteinemia

30. What aa becomes essential with phenylketonuria?
P53 and Rb; p53 activates BAX Which inhibits BCL2 When there is DNA damage - Rb phosphorylation allows progression into S phase
With glucose 6 phosphatase; Von Gierkes disease
Beta glucocerebrosidase; glucocerebroside
Tyrosine

31. What is the difference b/w beckers and duchennes muscular dystrophy?
Niacin treatment for hyperlipidemia; facial flushing
Beckers is less severe (instead of deletion is either defect or deficiency) with later presentation
Prolonged Ab use
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

32. What Ab class blocks DNA gyrase?
Flouroquinolones
Loss of subcutaneous fat - tissue and muscle wasting (somatic muscle); visceral - not somatic
Convulsions - hyperirritiability - peripheral neuropathy - sideroblastic anemias; alcoholics and INH and fortified goats milk consumption
Alpha ketoglutarate; no TCA cycle intermediates

33. What is the net production of glycolysis from one glucose?
Nacetyl cysteine; breaks disulfide bridges of mucus plugs
Gauchers; hepatosplenomegaly - aseptic necrosis of the femur - bone crises - Gauchers cells - crumpled tissue paper looking (fibrillar appearing macrophages in liver - spleen - b.m.)
Decreased substrate for gluconeogenesis (pyruvate to lactate and not oxaloacetate); and pyruvate doing only lactic acidosis and not glycolysis
2 NADH - 2 ATP - 2 pyruvate - 2H+ - 2H20

34. What is the inheritance pattern of hereditary hemorrhagic telangiectasia or Olser Weber Rendu Syndrome? What is it?
AD; disorder of blood vessels that results in telangietasias - skin discolorations - and AVMs (arteriovenous malformations)
By turning it into fructose - but sorbitol first; glucose to sorbitol via aldose reductase and sorbitol to fructose sorbital dehydrogenase
Beta thalassemia
Converts it to citrulline

35. What syndrome is characterized by hyperextensible skin and joints - and bleeding? What is the most common reason? What are other clinical symptoms?
Uses HMG coA reductase
Macroorchitism - big ears - big jaw - mental retardation (2nd mcc!) - mitral valve prolapse - long face
Biotin (vit B7) and Vit K
Ehler Danlos; type III collagen def; joint dislocation - berry aneurysm - ecchymoses - organ rupture

36. From What aa does GABA come from? with What enzyme? and What vit cofactor? What does GABA stand for?
5' end; 3' end
Delayed wound healing - hypogonadism - decreased adult hair - dysgeusia - anosmia - and alcoholic cirrhosis - perioral rash
Lactose in mothers milk contains galactose
Glutamate; glutamate decarboxylase; B6; gamma aminobutyrate

37. What are the titles of the two types of fructose disorders? Which is more serious>
Essential fructosuria and fructose intolerance; fructose intolerance
1. denature by heating 2. during cooling anneal with DNA primers 3. elongation - add heat stable DNA polymerase 4. repeat
Urea cycle enzyme deficiency (mc: ornithine transcarbamoylase); liver disease - alcoholic cirrhosis and reye's
Ortinithine transcarbamoylase - orotic acid phosphoribosyltransferase - and orotidine 5 phosphate decarboxylase; urea cycle; last two are involved in conversion of orotic acid to UMP (adding PRPP to orotic acid)

38. Only _____________ has no feedback imhibition
Phenylalanine build up
Meiotic nondisjunction in mothers gametes during the first meiosis
hexokinase - glucokinase (its a glutton)
Ouabain

39. What does topoisomerase do? I or II eukaryotic?
Lens and neural tissue; converts galactose to osmotically active galactitol
Microvilli - muscle contraction - cytokinesis - adherens junctions
Collections of abnormal mit collecting under sarcolemmal membranes and results in distortion of myofibrils
Creates nicks in DNA to prevent supercoiling; both!

40. How many bonds does a G- C connection have? What type of bonds? how about a A- T?
Pantothenate; essential comp of CoA (needed for acyl transfers) and fatty acid synthase
Carbamoyl phosphate to orotic acid
are AR
3 hydrogen bonds; 2 hydrogen bonds

41. Which cells are rich in SER?
Adrenal cells (steroid synthesis) - hepatocytes (p450 detox)
S. aureus - Aspergillus
Chromosomal imbalance (downs for ex)
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

42. What is a common result of unbalanced robertsonian translocation?
Chromosomal imbalance (downs for ex)
Fruits and vegetables
Galactose 1 phosphate uridyltransferase (GALT) results in an accumulation of toxic substances; failure to thrive - jaundice - hepatomegaly - infantile cataracts - mental retardation; exclude galactose and lactose from diet
pleiotropic

43. Amino acids are found in...
TCA cycle - FA oxidation - acetyl coA production; ox phos
Galactocerebrosidase; Galactocerebroside
only L form
Deficiencies in one of the many lysosomal enzymes results in accumulation of abnormal metabolic products

44. What happens to S adenosyl methionine after it donates a methyl? What enzyme regenerates methionine? What two factors are necessary to do this? who gives the methyl?
Deficiencies in one of the many lysosomal enzymes results in accumulation of abnormal metabolic products
S adenosyl homocysteine; homocysteine transferase; B12 and THF; M- THF
TRNA wobble
Lactose (glucose and galactose)

45. What is the most common human enzyme deficiency? why? What is the mode of inheritance of this deficiency?
Neimann pick and tay sachs
G6PD; provides malarial resistance; X linked recessive
Electron transport inhibitors - ATPase inhibitors - Uncoupling agents (increase permeability of the membrane - loss of hydrogen gradient)
Metanephrine

46. From What aa does histamine come from? with What vitamin cofactors help?
Urea cycle enzyme deficiency (mc: ornithine transcarbamoylase); liver disease - alcoholic cirrhosis and reye's
Lack of proper dietary therapy during pregnancy resulting in infant's microcephaly - mental retardation - growth retardation and congenital heart defects
Histidine; B6
normal lactate levels

47. Cysteine is one of two _____ connected by disulfide bond
dimers
By epoxide reductase in the liver; coumadin; intestinal flora and leafy vegetables
Nucleoside is just the ribose plus the base; nucleotide is the ribose plus the base but the phosphate bond
Developmental delay - gargoylism - airway obstruction (d/t short neck) - corneal clouding - CAD - hepatosplenomegaly

48. After citrulline is produced and in the cytoplasm What happens next in the urea cycle?
Catalase positive because they reduce their H2O2 and WBCs cant use that to make bleach; because the way they protect is by building a wall around the organisms with granulomas
Blood group types
Cycloheximide
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

49. ____________ disorders are worse than fructose disorders.
Galactose
5' end; 3' end
Riboflavin; dairy and meat
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 is significant about 50S/60S ribosomal unit?
Trinucleotide expansion diseases
It contains the peptidyltransferase activity (for elongation of protein
NADPH- reductive biosynthesis!
Pantothenate; essential comp of CoA (needed for acyl transfers) and fatty acid synthase

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USMLE Step 1 Biochemistry

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