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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 Abs block the binding between aminoacyl tRNA and rRNA by binding the 30S ribosome?
Aminoglycosides and tetracyclines
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)
TPP (thiamine pyrophosphate)
Phenylethanolamine N- methyl transferase; SAM

2. Loss of heterozygosity is the rule for...
A piece of glass or silicon chip with thousands of grids with different nucleic acid sequences and DNA or RNA probes are hybridized to the chip and a scanner detects the complementary binding; used to study a persons genome and to detect snps (single
Orotate
tumor suppressor genes (p53 - Rb - BAX)
Mental and growth retardation; seizures - fair skin - eczema - musty body odor; neurotoxic ketones and acids stimulate vomitting - can lead to pyloric stenosis

3. What is the mode of inheritance of Marfans?
Hemolytic anemia; jaundice in newborn if mom gets too much vit K
Orotate
AD
is asymmetric

4. What is the inheritance pattern of hereditary hemorrhagic telangiectasia or Olser Weber Rendu Syndrome? What is it?
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)
AD; disorder of blood vessels that results in telangietasias - skin discolorations - and AVMs (arteriovenous malformations)
Phospholipids - sphingolipids - glycolipids
activation of PFK1

5. What are the two main function of Vit E?
Both add phosphate; phosphorylase uses no ATP and inorganic phosphate
A cytoskeletal protein - apart of flagella - cilia - mitotic spindles and transport (in neurons for example)
Helps make Vit K and antioxidant
Which dimerizes and activates a protein phosphatase Which de phosphorylates glycogen phosphorylase

6. How do you treat fructose intolerance?
An anti cancer agent that is a topoisomerase II and creates nicks in DNA
are AR
Decrease intake of fructose and sucrose (fructose and glucose)
HMG CoA reductase; converts HMG CoA to mevalonate

7. What is the effect of defiiency of VIt E?
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
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
Increased homocysteine in the urine - mental retardation - osteoporosis - marfanoid habitus - and atherosclerosis
Down's Syndrome

8. What type of aa are histones made of? are they acidic or basic?
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
Assist in spontaneous refolding of proteins
I; nucleolus
Lysine and arginine; basic

9. How does fructose enter glycolysis? galactose?
DHF
Highly condensed forms of DNA; mitosis; neoplasia - highly dividing and thus hyperchromatic
Kidney and intestinal epithelium
Via fructose 1 P by fructokinase in the liver; as glucose 1 by being GALT- ed ;)

10. What are the two most common aminotransferase and What amino acids do they turn to What glycolytic intermediates?
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
Alanine aminotransferase (ALT) and aspartate aminotransferase (AST); alanine to pyruvate and aspartate to oxaxloacetate
Pataus
Lyonization; nondisjunction in mitotic divisions during embryonic period

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

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12. Which Hurlers is milder and presents with no corneal clouding?
Mental retardation - rocker bottom feet - micropthalmia - microcephaly - cleft lip/palate - holoprosoncephaly - polydactyly
Chediak Higashi Syndrome; decreased phagocytosis results in recurrent pyogenic infections - partial albinism - and peripheral neuropathy
Phosphorylation of glucose
XR - iduronate sulfatase

13. How does insulin both deactivate glycogenolysis and activates glycogenesis at same time?
By activating protein phosphatase Which both dephosphorylates glycogen phosphorylase (deactivating) and dephosphorylates glycogen synthase (activating it )
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
Electron transport inhibitors - ATPase inhibitors - Uncoupling agents (increase permeability of the membrane - loss of hydrogen gradient)
Avidin in raw egg whites prevents their reabsorption; also antibiotic use; dermatitis - alopecia - glossitis - lactic acidosis - enteritis

14. Are histones in heterochromatin or euchromatin more methylated? Which had more acetylated histones?
Heterochromatin; euchromatin
Schwann cells - lens - retina - kidneys because they only have aldose reductase and no sorbital dehydrogenase
Enriched whole grain cereals and other things; thiamine; malnutrition (nonenriched white processed bread and rice) and alcohol second to malnutrition
Cardiomegaly and systemic findings leading to early death; lysosomal alpha -1 -4- glucosidase (acid maltase)

15. 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?
Mental retardation - excessive laughter - seizures - ataxia - 'happy puppet'
Glutamate; glutamate decarboxylase; B6; gamma aminobutyrate
Glycerol and propionyl coA; lactate and alanine
Dihydrofolate reductase; vitamin B12; homocysteine

16. What 3 genetic disorders can cause Marfanoid habitus?
Def in Vit C; def in Vit D
Pulmonary - GI - reproductive (infertility in male due to absence of vas deferens and seminal fluid thickness)
Arginne
Marfans - homocystinuria - MEN 2B

17. What is the deficient enzyme in Fabry's disease? accumulated Substrate?
Peripheral neuropathies of hands/feet; angiokeratomas between umbilicus and knees; cardiovascular/renal disease
Convulsions - hyperirritiability - peripheral neuropathy - sideroblastic anemias; alcoholics and INH and fortified goats milk consumption
Bone - Skin - Tendon - dentin - fascia - cornea - late wound repair
Alpha galactosidase A; ceramide trihexoside

18. What is the deficient enzyme in Tay Sachs disease? accumulated Substrate?
Branche aa alpha ketoacid dehydrogenase; thiamine; Isoleucine - leucine - valine
Hexosaminidase A; GM2 ganglioside
Self mutilation - aggression - mental retardation - gout - hyperuricemia - choreathetosis
Wrinkles - acne and psoriasis topically - promyelocytic anemia - measles

19. What does 6 mercaptopurine do?
Lesch Nyhan syndrome; X linked recessive
Glutamine - aspartate - glycine; aspartate
6MP blocks de novo purine synthesis
Sulfonamides - primaquine - dapsone - antiTB drugs - fava beans - LDL; RBCs

20. What type of disorders are associated with chromosomes 13 -14 -15 -21 -22?
Robertsonian translocations
Tyrosinase deficiency; defective tyrosine transporters; lack of migration of neural crest cells; skin cancer
Mitochondria in liver; HmG CoA synthetase; amino acids and beta oxidation of FA
Beckers is less severe (instead of deletion is either defect or deficiency) with later presentation

21. why does galactose def present in newborns>
Induction in chronic alcoholism (increases tolerance); inhibition in acute alcoholism
Progressive neurodegeneration (spasticity d/t UMN disease) - hepatosplenomegaly - cherry red spot on macula - foam cells
Lactose in mothers milk contains galactose
Pigmented iris hamartomas

22. From What is pyruvate produced in glycolysis? but What enzyme? What enhances that enzyme? downregulates it?
Carnitine acyltransferase I
Differentiation of cells - vision in reduced light (retinol makes up rhodopsin cells) - antioxidant
Edwards
Phosphoenolpyruvate; pyruvate kinase; fructose 1 - 6 BP; ATP - alanine

23. What is the precursor in the skin that the sun turns to D3?
7 dehydrocholesterol
acetyl coA
Tyrosine; tryptophan
Neural tissue and adrenal medulla

24. fatty acid synthase requires ...
Yes! congenital simply means 'born with'- hereditary is familial
NADPH- reductive biosynthesis!
Carnitine acyltransferase I
Lack of proper dietary therapy during pregnancy resulting in infant's microcephaly - mental retardation - growth retardation and congenital heart defects

25. Which form of iron is most absorbable? What form is non heme ingested iron in (vegetables for ex)?
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
Fe2+; Fe3+
Met - arg - val - his
hexokinase (glucokinase only found in liver and beta cells of pancreas)

26. What enzyme is deficient in Fructose intolerance? What is the result of the enzyme def? What are the symptoms?
Aldolase B; Fructose 1 phosphate accumulates Which uses up Phosphate and the result is inhibition of gluconeogenesis without phosphate; hypoglycemia - jaundice - cirrhosis - vomitting
Stored ATP - creatine phosphate - anaerobic glycolysis; as distances increase - ATP is obtained from additional resources; above plus ox phos
Nacetyl cysteine; breaks disulfide bridges of mucus plugs
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

27. RER adds ___________ to asparagines
2 NADH - 2 ATP - 2 pyruvate - 2H+ - 2H20
Dopamine beta hydroxylase; vit C
N linked oligosaccharides
Succinyl coA

28. What disease is associated with chromosome 3?
VHL (Von Hippa Lindau)
Cleft palate - Abnormal faces - thymic aplasia (t cell def) - Cardiac defects - hypocalcemia (secondary to hypoparathyroid); 3rd and 4th branchial pouch aberrant development
So that excess glucose can be stored (phosphorylated glucose cant leave) in the liver and saved as a buffer
Complex I - Complex II (lower energy)

29. Which RNA polymerase synthesizes tRNAs and snRPs?
APCKD
When there is glucagon; protein kinase A activation and thus phosphorylation of the complex resulting in the activation of fructose 2 -6 BPase and no PFK2
AR
III

30. What is rickets? What is it caused by?
Bendy bones seen in children due to decreased Vit D; craniotabes - skeletal deformities - growth retardation - rosary
Pompes
Glucose 6 phosphate dehydrogenase
Homocysteine methyltransferase - cystathionine reductase - decreased affinity of cystathionine reductase for pyridoxal phosphate; AR

31. What is the degradation product of Norepinephrine?
RBCs - renal medulla - leukocytes - lens - testes - cornea
VMA
Carbidopa
To provide NADPH since there is an abundance of glucose 6 phosphate; also yields ribose for nucleotide synthesis and glycolysis intermediates

32. What inheritance pattern is Type IIA hypercholesterolemia? do heterozygotes present with disease? do homozygotes present?
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
Vitamin B1 (thiamine) - Vitamin B2 (riboflacin) - Vitamin B3 (niacin) - Vitamin B5 (pantothenate) - Vitamin B6 (pyridoxine) - Vitamin B7 (Biotin) - Vitamin B12 (cobalamin) - folate - Vit. C
4 -4 -9
AD: yes! most common presentation; very rare (700 mg/dl cholesterol level!)

33. What enzyme is deficient in maple syrup disease? What does it result in?
Intestine - renal - bone
HVA
dimers
Branched chain alpha keto acid dehydrogenase; inability to break down branched chain aa

34. What are two reasons why pyruvate would be pushed to lactate in an alcoholic?
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
Metabolizes them with transferase enzymes to acetyl coA
Increased NADH causes lactate production and pyruvate dehydrogenase def causes increased lactate (d/t thiamine def that often occurs with alcoholics)
Electron transport inhibitors - ATPase inhibitors - Uncoupling agents (increase permeability of the membrane - loss of hydrogen gradient)

35. Kwashiokor patients are not...
Using serine (which turns into glycine after reaction)
calorie deficient
Decrease intake of fructose and sucrose (fructose and glucose)
Rb

36. What is given to prevent side effects When methotrexate is given? why does this work?
'9+2' 9 pairs of microtubules connected by an axonemal dynein ATPase in a cylinder with 20 MTs in the center; the connecting ATPase allows bending and differential sliding
AD; disorder of blood vessels that results in telangietasias - skin discolorations - and AVMs (arteriovenous malformations)
Inhibits lipoic acid; lipoic acid is a cofactor for pyruvate dehydrogenase
Folinic acid; does not require DHF

37. What can cause a vit C deficiency?
Folinic acid; does not require DHF
Leucine and lysine
Smoking (FR takes up all the antioxidants); diets lacking fruit
Testing for antigen - antibody reactivity; can either add an antibody to a persons blood sample to see if there is binding or add an antigen to a persons blood sample to see if there immune system recognizes it; close to 100% for both; HIV eliza - an

38. What are the findings in Metachromatic leukodystrophy?
Glucokinase
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)
I; nucleolus
Central and peripheral demyelination with ataxia and dementia

39. What causes an excess of B3? What are the symptoms?
Pyruvate! (not lactate - that is generate just in order to produce more NAD+)
Niacin treatment for hyperlipidemia; facial flushing
Cystic Fibrosis (CFTR gene)
AR

40. 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
Cafe au lait spots - optic pathway gliomas - lisch nodules - scoliosis - neural tumors
Pyruvate dehydrogenase; ATP - NADH - acetyl CoA
Arylsulfatase A; cerebroside sulfate

41. Other than the H2O2 produced from the resp burst in infections - What other things does G6PD reproduce NADPH for glutathione reduction for? What cells are particularly vulnerable to these oxidizing agents?
Sulfonamides - primaquine - dapsone - antiTB drugs - fava beans - LDL; RBCs
show disease
Fructose bisphosphatase 2 and Phosphofructokinase 2; phosphorylation by protein kinase A; inuslin/glucagon ratio
S - adenosyl methionine

42. What toxins prevent elongation in translation in humans (by binding 40s)?
Dopa decarboxylase; B6
Shiga toxin and ricin (protein in castor beans)
tubulin dimer
By giving high amounts of pyridoxal phosphate in the diet

43. What is the other name of B3? What is its function? Synthesis of B3 requires What other vitamin? What else?
3'--> 5'; prokaryote only
Niacin/Nicotinic acid; NAD+ and NADH used in redox reactions - and catabolic and anabolic reactions; B6; tryptophan
Shiga toxin and ricin (protein in castor beans)
They can enter G1 from G0 and replicate if they like; hepatocytes - lymphocytes - fibroblasts

44. What is the fate of the two products of lipolysis?
Myotonic Dystrophy (CTG) - Huntington (CAG) Friedricks Ataxia (GAA) - Fragile (CGG)
TFs
Glycerol goes to liver for gluconeogenesis; free fatty acids bind to albumin in blood and travel to cells to be used as for energy
osteosarcomas (Rb mutation is in all cells)

45. What are two examples of X linked Autosomal Dominant disease?
In the charging reaction! for this reason aminoacyl tRNA synthetase double checks
Alanine (through pyruvate) - aspartate (through OXA) - glutamate (alpha ketoglutarase); TCA
Hypophosphatemia rickets and Alports
fructose

46. What is etoposide? why does this not affect normal healthy cells?
Dilated capillaries and veins; skin - mucous membranes - GI - resp - urinary tracts; rupture and can cause epistaxis - hemorrhage - GI bleeding
Lens and neural tissue; converts galactose to osmotically active galactitol
High glycogen in muscle - but cannot break it down - leading to painful muscle cramps - myoglobinuria with strenuous exercise - no increase in lactic acid after exercise; skeletal muscle glycogen phophorylase
A topoisomerase II inhibitor; because cancer cells use II more so than healthy cells

47. What enzyme converts norepinephrine to epinephrine? using What cofactor?
TPP (from B1- pyrophosphate) - FAD (B2) - NADH2 (B3) - CoA (B5 - panothenate) - Lipoic Acid
Prevents neural tube defects in 1st trimester
Phenylethanolamine N- methyl transferase; SAM
Gain of function gene after expansion results in overproduction of huntingtin protein that in overproduction results in toxicity to neurons in the caudate --> caudate atrophy and decreased levels of GABA and Ach

48. What causes fat soluble vitamin deficiencies?
Malabsorption problems (CF and sprue) and mineral oil intake
Fructose bisphosphatase 2 and Phosphofructokinase 2; phosphorylation by protein kinase A; inuslin/glucagon ratio
Acetyl coA activated pyruvate carboxylase in gluconeogenesis
Dihydrofolate reductase; vitamin B12; homocysteine

49. What are the clinical findings in patau syndrome?
Degeneration of retinal ganglion cells and axons; acute loss of central vision; mit. inheritance
Calcium and calmodulin activate glycogen phosphorylase kinase
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
Mental retardation - rocker bottom feet - micropthalmia - microcephaly - cleft lip/palate - holoprosoncephaly - polydactyly

50. What two substrates (other than pyruvate) does pyruvate carboxylase require?
Fructose gets phosphorylated into fructose 1- P by fructokinase and then fructose 1P gets metabolized into DHAP or glyceraldehyde by aldolase B
Central and peripheral demyelination with ataxia and dementia
ATP and biotin
Carbidopa

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USMLE Step 1 Biochemistry

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