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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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Match each statement with the correct term.
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1. What is the difference between exonucleases and endonucleases?
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
S adenosyl homocysteine; homocysteine transferase; B12 and THF; M- THF
Severe fasting hypoglycemia - very high levels glycogen in liver - high blood lactate - hepatomegaly
Endonucleases are specific and break specific linking phosphodiester bonds - exonucleases are not sequence specific and just remove the nucleoside

2. What do hexokinase and glucokinase do? using what? What pathway is this in?
'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
Cycloheximide
A ketone
Phosphorylation of glucose to yield glucose 6 phosphate; ATP; first step of glycolysis

3. What disease is associated with chromosome 5?
Cri du Chat
1 day; adipose release of FFA - hepatic gluconeogenesis from peripheral tissue lactate and alanine and from adipose tissue glycerol and propionyl coA
Via fructose 1 P by fructokinase in the liver; as glucose 1 by being GALT- ed ;)
Disulfiram (antabuse); acetaldehyde accumulate contributing to hangover

4. What is the source of ATP When you are fasting (in between meals)?
Limit dextrans
be anorexic
Vitamin B1 (thiamine) - Vitamin B2 (riboflacin) - Vitamin B3 (niacin) - Vitamin B5 (pantothenate) - Vitamin B6 (pyridoxine) - Vitamin B7 (Biotin) - Vitamin B12 (cobalamin) - folate - Vit. C
Hepatic glycogenolysis (mostly) - also hepatic gluconeogenesis - adipose release of FFA (minor)

5. What is the reason for blue sclerae in Osteogenesis Imperfecta?
Lens and neural tissue; converts galactose to osmotically active galactitol
Thinned cornea from lack of connective tissue so see underlying choroidal lens
A cytoskeletal protein - apart of flagella - cilia - mitotic spindles and transport (in neurons for example)
Tryptophan; niacin

6. What is needed for glycolysis to continue? How is this provided in aerobic glycolysis? anaerobic?
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
Decrease intake of fructose and sucrose (fructose and glucose)
Aldose reductase in the lens converts built up galactose to galactitol Which is osmotically active

7. Other than thymidylate synthase - What else is required to convert dUMP to dTMP?
activator
Neutrophils and monocytes; membrane bound; immune response by generating Reactive oxygen species
N5N10 methylene THF
kidney

8. Which amino acid is the most basic?
Liver mit and liver cytosol
Arg
To increase serum Ca and phosphate for bone deposition
A 16C FA! the product in FA synthesis; saturated; no double bonds

9. why does a deficiency in ornithine transcarbamoylase (urea cycle enzyme) result in an increased production of orotic acid - the pyrimidine precursor?
Because carbamaoyl phosphate is involved in both urea cycle and pyrimidine synthesis
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
Gets converted to pyruvate via the Cori cycle in the liver
Arginine

10. AD diseases are usually...
plasma membrane
Milder form of type 1 with normal blood lactate levels; debranching enzyme (alpha -1 -6 glucosidase)
pleiotropic
In the charging reaction! for this reason aminoacyl tRNA synthetase double checks

11. How do digoxin and digitoxin work? What class of drugs are they?
Osteoblasts; de phosphorylates pyrophosphate Which inhibits bone mineralization; bone mineralization
Back up of pyruvate and alanine and thus lactic acidosis occurs; can be d/t alcoholism (vit B1 def)
To supply sufficient glucose to the brain and the RBCs and to preserve protein
Inbit Na+K+ ATPase; Cardiac glycosides

12. From What aa does porphyrin come from? What is porphyrin the precursor of?
N5N10 methylene THF
Essential fructosuria and fructose intolerance; fructose intolerance
Glycine; heme
Pyruvate to alanine (to carry amino groups to liver) - pyruvate to OAA for gluconeogenesis (or replenish for TCA cycle) - acetyl coA (for TCA cycle transition or FA or chol. synthesis) - or the end of anaerobic glycolysis

13. What 3 genetic disorders can cause Marfanoid habitus?
Galactocerebrosidase; Galactocerebroside
Dermatitis - glossitis - diarrhea
Glycerol and propionyl coA; lactate and alanine
Marfans - homocystinuria - MEN 2B

14. What luminal secretions are affected in CF? What is wrong with the secretions?
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
Respiratory - seminal fluid - pancreatic - gallbladder; very thick without NaCl to pull water
Isoleucine - valine - leucine
Biotin; rate limiting step in fA synthesis - acetyl coA into malonyl coA

15. How do you treat orotic aciduria? How does this work?
Helps make Vit K and antioxidant
Preprocollagen - procollagen - tropocollagen - collagen
Oral uridine administration; provides nucleosides and provides feedback inhibition
To transfer methyls; a methionine and an ATP; norepinephrine plus a methyl makes epinephrine

16. What are the functions of Microtubule proteins?
Glucokinase
Fructose 1 -6 bisphosphatase
A cytoskeletal protein - apart of flagella - cilia - mitotic spindles and transport (in neurons for example)
glycogenolysis

17. What are the clinical symptoms of orotic aciduria?
Mucus secreting goblet cells - ab secreting plasma cells - nt secreting neurons
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)
degradation
Increased orotic acid in urine - megaloblastic anemia - +/- hyperammonemia

18. What does SAM turn into after it loses a methyl? How does it get turned back to SAM?
Cell signaling defect in FGF receptor resulting in dwarfism and short limbs with normal head and trunk; AD
Kartaganer's aka ciliar dyskinesia; lack of dynein arm ATPase
TCA cycle - FA oxidation - acetyl coA production; ox phos
Homocysteine; homocysteine methyltransferase with B12 and tetrahydrofolate

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

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20. What is the rate limiting step in glycolysis? What does it do? who activates it? inhibits it?
Later
Pantothenate; essential comp of CoA (needed for acyl transfers) and fatty acid synthase
Phosphofructokinase 1; fructose 1 phosphate to fructose 1 -6 bisphosphate; enhancers: AMP - fructose 2 -6 Bisphosphate; downregulators: citrate - ATP
Inbit Na+K+ ATPase; Cardiac glycosides

21. What is a positive allosteric regulator on pyruvate carboxylase? for What reaction is this?
RER in nucleus (synthesize NTs)
Thymidylate synthase; 5- Flourouracil
Acetyl coA; gluconeogenesis
calorie deficient

22. What is the fate of glucose from glycogenolysis?
fat
Carnitine shuttle
Make it stiffer/more tensile; continues as we get older so gets less and less elastic
Glucosekinase to glucose 6 P

23. What is the fate of the two products of lipolysis?
all carriers
Neural tissue and adrenal medulla
teratogenic
Glycerol goes to liver for gluconeogenesis; free fatty acids bind to albumin in blood and travel to cells to be used as for energy

24. other than in the kidney - Where else are gluconeogenesis enzymes found?
Kidney and intestinal epithelium
Lack of lactase (brush border enzyme) so can not break down lactose (a dissaccharide of galactose and glucose); African Americans and Asians
Phenylalanine hydroxylase or tetrahydrobiopterin factor
Arg - lys - his; positive except His has no charge at body pH

25. Prokaryotes have What ribosomal units?
30S + 50S= 70S
Increased TG synthesis from increased Glycerol 3 P and abetalipoproteinemia
5'--> 3'; prokaryote only; for removing RNA primer and filling it with DNA
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)

26. How is muscular dystrophies diagnosed?
Reserve supply of glucose in the fasting state - liver gives to the blood and skeletal muscle uses for itself
Niacin treatment for hyperlipidemia; facial flushing
Cartilage (including hyaline) - vitreous body - nucleus pulposus
Increased CPK and muscle biopsy

27. What vitamin is given at birth to prevent hemorrhage? why?
severe
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
Vit K; neonates have sterile intestine and can not synthesize vit K
Collagen; fibroblasts; 4; ECM

28. What does a pyridoxine deficiency look like? What causes it?
Tyrosine hydroxylase; tetrahydrobiopterin factor; NADP+
Convulsions - hyperirritiability - peripheral neuropathy - sideroblastic anemias; alcoholics and INH and fortified goats milk consumption
Gauchers; hepatosplenomegaly - aseptic necrosis of the femur - bone crises - Gauchers cells - crumpled tissue paper looking (fibrillar appearing macrophages in liver - spleen - b.m.)
In the charging reaction! for this reason aminoacyl tRNA synthetase double checks

29. From What aa does glutathione come from?
Glutamate
Conversion to UDP glucose via UDP glucose pyrophosphorylase; glycogen synthase (rate limiting step )
Glycine; proline and lysine
Lack of proper dietary therapy during pregnancy resulting in infant's microcephaly - mental retardation - growth retardation and congenital heart defects

30. Because electron transport continues...
O2 consumption (last electron acceptor) gets consumed also
Decreased phenylalanine - increase tyrosine
Cytosine to uracil
Mucus secreting goblet cells - ab secreting plasma cells - nt secreting neurons

31. Amino acids are found in...
Pyruvate carboxylase (pyruvate to OXA) (mit) - PEP carboxykinase (OXA to PEP) (cyt) - Fructose 1 -6 bisphophatase (fructose 1 -6 bisphosphate to fructose 6 P) (cyt) and glucose 6 phosphatase (in ER); glucose 6 phosphatase
only L form
Ornithine transcarbomyalse; orotic aciduria; X linked recessive; autosomal recessive
In the charging reaction! for this reason aminoacyl tRNA synthetase double checks

32. What disease is associated with chromosome 18?
Increased orotic acid in urine - megaloblastic anemia - +/- hyperammonemia
Edwards
Heterochromatin; euchromatin
Uric acidemia; HGPRT; purine salvage

33. What is the rate determining enzyme in fatty acid oxidation?
Pulmonary - GI - reproductive (infertility in male due to absence of vas deferens and seminal fluid thickness)
Dynein - retrograde - kinesin - anterograde
5' capping (with 7 methylguanosine) - 3' adenylation - splicing of introns; nucleus
Carnitine acyltransferase I

34. What enzyme is deficient in galactokinase deficiency? What are the symptoms?
Collections of abnormal mit collecting under sarcolemmal membranes and results in distortion of myofibrils
Galactokinase; galactose appears in blood and urine - sometimes infantile cataracts may present as failure to track objects or lack of a social smile
Cycloheximide
Limit dextrans

35. What regulates Fructose 1 -6 bisphosphatase? What reaction is it found in?
Antineoplastic agents; alkylate DNA!; cyclophosphamide and nitrosurea (aka lomustrine - a nitrosource)
B12 and folate; B12; washed out
Citrate is positive on it - AMP and fructose 2 -6 bisphosphate is negative on it; gluconeogenesis
Arginine

36. What are the lab findings see in hyperammonemia?
10 - 9 - 7 - 2; protein C and S
glycogen
Increased blood levels of ammonia - decreased BUN
Uses HMG coA reductase

37. What can cause Biotin deficiency? What are the symptoms?
2 NADH - 2 ATP - 2 pyruvate - 2H+ - 2H20
hexokinase - glucokinase (its a glutton)
Avidin in raw egg whites prevents their reabsorption; also antibiotic use; dermatitis - alopecia - glossitis - lactic acidosis - enteritis
Meconium ileus and failure to thrive; negative serum immunoreactive trypsin

38. other than bone resorption to release Ca and phosphate - What is the ultimate point of increasing the serum Ca2+ and phosphate?
Reserve supply of glucose in the fasting state - liver gives to the blood and skeletal muscle uses for itself
Hexosaminidase A; GM2 ganglioside
tubulin dimer
To increase serum Ca and phosphate for bone deposition

39. What are the three types of blotting procedures ? What are blotting procedures used for? What are steps in a blotting procedure?
Williams
Carbamoyl phosphate to orotic acid
Lys and arg
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

40. Which is the rate limiting enzyme in ethanol metabolism? Where is this enzyme located? Where is the other enzyme located? What is the limiting reagent?
Steroid synthesis and detoxification
Alcohol dehydrogenase; cytosol; mitochondria; NAD+
alk phos
MT!

41. In What syndrome is HGPRT deficient? What is the inheritance pattern?
A topoisomerase II inhibitor; because cancer cells use II more so than healthy cells
Lesch Nyhan syndrome; X linked recessive
Galactokinase; galactose appears in blood and urine - sometimes infantile cataracts may present as failure to track objects or lack of a social smile
Defective neutral amino acid transporter on renal and intestinal cells; AR

42. What is the pathogenesis of skurvy? name some clinical symptoms.
hexokinase - glucokinase (its a glutton)
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
Down's Syndrome
Developmental delay - gargoylism - airway obstruction (d/t short neck) - corneal clouding - CAD - hepatosplenomegaly

43. What is the most common human enzyme deficiency? why? What is the mode of inheritance of this deficiency?
G6PD; provides malarial resistance; X linked recessive
Bacteria produce lactate acid and gases from lactose and produce acidic diarrhea and bloating
Conversion to UDP glucose via UDP glucose pyrophosphorylase; glycogen synthase (rate limiting step )
Schwann cells - lens - retina - kidneys because they only have aldose reductase and no sorbital dehydrogenase

44. What aa becomes essential with phenylketonuria?
Chediak Higashi Syndrome; decreased phagocytosis results in recurrent pyogenic infections - partial albinism - and peripheral neuropathy
HMG CoA reductase
Tyrosine
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

45. What is the rate determining enzyme in de novo pyrimidine synthesis?
Chediak Higashi Syndrome; decreased phagocytosis results in recurrent pyogenic infections - partial albinism - and peripheral neuropathy
5'--> 3'; prokaryote only; for removing RNA primer and filling it with DNA
Cafe au lait spots - optic pathway gliomas - lisch nodules - scoliosis - neural tumors
Carbomyl phosphate synthetase II

46. What does PEP carboxykinase require? and for What reaction is this?
All and Alzheimers
GTP: gluconeogenesis - OXA to phosphoenolpyruvate
DOPA
Kartaganer's aka ciliar dyskinesia; lack of dynein arm ATPase

47. After addition of an aa - Where does the aminoacyl tRNA now bind to?
Lime and CaCO3 to displace the niacin
30S ribosomal RNA (small one)
Avidin in raw egg whites prevents their reabsorption; also antibiotic use; dermatitis - alopecia - glossitis - lactic acidosis - enteritis
linked recessive

48. What is the deficient enzyme in Krabbes disease? accumulated Substrate?
Dark connective tissue - brown pigmented sclera - debilitating athralgias; homogentistic acid builds up and can be toxic to cartilage
Succinyl coA
Chediak Higashi Syndrome; decreased phagocytosis results in recurrent pyogenic infections - partial albinism - and peripheral neuropathy
Galactocerebrosidase; Galactocerebroside

49. After day 3 of starvation - What is the main source of energy?
By cyclin CDK complexes
Adipose tissue for brain and heart (ketone bodies) and then go to vital protein degradation (organ failure)
Cafe au lait spots - optic pathway gliomas - lisch nodules - scoliosis - neural tumors
Delayed wound healing - hypogonadism - decreased adult hair - dysgeusia - anosmia - and alcoholic cirrhosis - perioral rash

50. What is DNA gyrase?
Abnormal glycogen metabolism results in accumulation of glycogen in cells
Aldolase B
A topoisomerase II
DOPA

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USMLE Step 1 Biochemistry

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