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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 creates a frameshift mutation? What are some clinical examples?
Insertion or deletion of a number of nucleotides indivisible by 3; tay sachs - duchennes MD
TFs
Preprocollagen - procollagen - tropocollagen - collagen
P53 and Rb; p53 activates BAX Which inhibits BCL2 When there is DNA damage - Rb phosphorylation allows progression into S phase

2. What is the degradation product of epinephrine?
Metanephrine
Complex I - Complex II (lower energy)
Glycolysis - fatty acid synthesis - HMP shunt - protein synthesis (RER) - steroid synthesis (SER) - glycogenolysis - glycogenesis
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

3. What does Vit D stimulation of osteoblasts do?
Release alk phos
When child gets two chromosomes from one parent; imprinting dx
hexokinase (glucokinase only found in liver and beta cells of pancreas)
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

4. Which catecholamine has inhibitory action against acetylcholine?
SAM
Its the ribose; its made from PRPP synthetase using ribose 5 P from the HMP shunt
Cell signaling defect in FGF receptor resulting in dwarfism and short limbs with normal head and trunk; AD
Dopamine

5. What is the main use of glycogen?
Reserve supply of glucose in the fasting state - liver gives to the blood and skeletal muscle uses for itself
Vit C deficiency; glossitis - anemia - weakened vessels - hemorrahges - hemarthroses - swollen gums - bruises -
3 hydrogen bonds; 2 hydrogen bonds
only L form

6. What is another way to trap glucose in cells but not by phosphorylating it? What enzymes are used?
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
TFs
By turning it into fructose - but sorbitol first; glucose to sorbitol via aldose reductase and sorbitol to fructose sorbital dehydrogenase
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

7. What is the order of cell cycle phases? What happens in each phase?
Vit C
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)
Dactinomycin - doxorubicin
Nucleoside is just the ribose plus the base; nucleotide is the ribose plus the base but the phosphate bond

8. Which TCA cycle enzyme is found in the inner mit membrane? Where are the rest found?
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
Succinate hydrogenase; matrix
Immediately after replication - DNA is unmethylated and so it is a signal to be double checked; HNPCC; heterochromatin
Orotate

9. On a 100 meter sprint What is used as energy? in general What is the rule with exercise? What is used in 1000 meter run?
liver and muscle
Stored ATP - creatine phosphate - anaerobic glycolysis; as distances increase - ATP is obtained from additional resources; above plus ox phos
Tyrosinase
glucagon - insulin and epinephrine

10. What is cystinuria due to?
Hereditary defect of renal tubular amino acid transporter for cysteine - ornithine - lysine and arginine in the PCT of kidnyes
10 - 9 - 7 - 2; protein C and S
Vit C; Vit E
anabolic processes

11. 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?
Pellagra (dermatitis - diarrhea - dementia); INH use - B6 deficiency - Hartnup disease - malignant carcinoid syndrome - and a corn based diet (lacks trp and niacin in corn cant be absorbed)
Nitric Oxide - creatinine - Urea
Electron transport inhibitors - ATPase inhibitors - Uncoupling agents (increase permeability of the membrane - loss of hydrogen gradient)
Macrocytic megaloblastic anemia; it is the most common vit def in the US; no neuro symptoms and no methylmalonyl coA in the urine

12. What lysosomal storage diseases are seen more in Ashkenazi Jew?
Induction in chronic alcoholism (increases tolerance); inhibition in acute alcoholism
ATP synthase - ATP- ADP translocase - ETC
Tay Sachs - Niemann Picks - Gauchers
Helps make Vit K and antioxidant

13. What is the treatment for cystinuria?
It induces its action!
VHL (Von Hippa Lindau)
Excreted from the kidney in the form of urea in hepatocytes
Acetazolamide to alkalinize the urine

14. What drug blocks ribonucleotide reductase? What does this enzyme do?
Lyonization; nondisjunction in mitotic divisions during embryonic period
Vit C
Von gierkes
Hydroxurea; deoxygenates ribose

15. What are some catalase positive bugs?
S. aureus - Aspergillus
Pulmonary - GI - reproductive (infertility in male due to absence of vas deferens and seminal fluid thickness)
Cortisol; inhibits insulin action
glucagon - insulin and epinephrine

16. What carries activated Phosphate groups?
AD
ATP
Cycloheximide
An AP (apyrimidinic) endonuclease to cut the DNA - exonuclease then removes and then filled and sealed by enzymes

17. What else decreases production of oxaloacetate in alcoholism other than decreased pyruvate?
calorie deficient
Vit C; Vit E
Oxaloacetate gets shifted to malate d/t excess NADH
Fabrys Disease and Hunters Syndrome

18. What does alcohol do to the P450 system?
Induction in chronic alcoholism (increases tolerance); inhibition in acute alcoholism
TCA cycle - FA oxidation - acetyl coA production; ox phos
Alkaptonuria; ochronosis
7 dehydrocholesterol

19. Insulin acts on __________ to deactivate glycogenolysis
both liver and muscle (unlike glucagon)
HMG CoA reductase; converts HMG CoA to mevalonate
aldose reductase
HMP shunt (to NADPH) and glycogenolysis (as glucose 1 phosphate)

20. What converts dopamine to norepinephrine? using What cofactor?
To provide NADPH since there is an abundance of glucose 6 phosphate; also yields ribose for nucleotide synthesis and glycolysis intermediates
By cyclin CDK complexes
Macrocytic megaloblastic anemia; it is the most common vit def in the US; no neuro symptoms and no methylmalonyl coA in the urine
Dopamine beta hydroxylase; vit C

21. What causes liver damage in kwashiokor?
usually present late
N5N10 methylene THF
Increased TG synthesis from increased Glycerol 3 P and abetalipoproteinemia
Electron transport inhibitors - ATPase inhibitors - Uncoupling agents (increase permeability of the membrane - loss of hydrogen gradient)

22. How many enzymes does the pyruvate dehydrogenase complex have? and How many cofactors? Where is it located? to What enzyme is it similar to?
Water soluble vitamin stored in liver; converted to THF by dihydrofolate reductase and is a coenzyme in 1 methyl transfers - impt for DNA synth
In hepatocytes; B6
3; 5; mit matrix; alpha ketoglutarate dehydrogenase complex
Kartaganer's aka ciliar dyskinesia; lack of dynein arm ATPase

23. What is the inheritance of Von Hippel Lindau disease?
hexokinase (glucokinase only found in liver and beta cells of pancreas)
AD
Cell signaling defect in FGF receptor resulting in dwarfism and short limbs with normal head and trunk; AD
Heteroplasmy

24. What causes hypoglycemia in alcoholism?
Robertsonian translocations
Decreased substrate for gluconeogenesis (pyruvate to lactate and not oxaloacetate); and pyruvate doing only lactic acidosis and not glycolysis
Highly condensed forms of DNA; mitosis; neoplasia - highly dividing and thus hyperchromatic
Respiratory - seminal fluid - pancreatic - gallbladder; very thick without NaCl to pull water

25. What are the glucogenic and ketogenic amino acids?
Increased homocysteine in the urine - mental retardation - osteoporosis - marfanoid habitus - and atherosclerosis
COP II
AST (mit damage) - and GGT (d/t SER hyperplasia d/t induction of p450 generation)
Ile - Phe - Thr - Trp

26. What is the source of ATP When you are fasting (in between meals)?
Make it stiffer/more tensile; continues as we get older so gets less and less elastic
Creates nicks in DNA to prevent supercoiling; both!
Hepatic glycogenolysis (mostly) - also hepatic gluconeogenesis - adipose release of FFA (minor)
teratogenic

27. What builds up in the urine in homogentistic acid oxidase deficiency? What does this result in ?
Cardiomegaly and systemic findings leading to early death; lysosomal alpha -1 -4- glucosidase (acid maltase)
Urine turns black on standing
Arginne
the RER

28. What are uncoupling agents? What is the result?
They increase the permeability of the inner mit membrane resulting in a loss of gradient and loss of ATP production BUT electron transport continues! heat is generated instead
To amplify a specific desired fragment of DNA
Self mutilation - aggression - mental retardation - gout - hyperuricemia - choreathetosis
5'--> 3'; prokaryote only; for removing RNA primer and filling it with DNA

29. From What aa does dopa come from?
DOPA
Tyrosine
fatty acid degradation
NF2 - DiGeorge/Velocardiofacial syndromes

30. What aa does tyrosine come from?
Vimentin - desmin - cytokeratin - glial fibrillary acidic protein (GFAP) - neurofilaments; to differentiate between different cells
It inhibits conversion of DHF to THF by blocking dihyrdofolate reductase
Phenylaline
is asymmetric

31. From What aa do the catecholamines come from? and What aa does that aa come from?
Respiratory burst - P450 - and glutathione reductase
Cylindrical made up of a helical array of polymerized alpha and beta tubulin dimers - each dimer has 2 GTP bound
Galactose
Tyrosine; phenylaline

32. What are two clinical symptoms of newborns with CF? and whats a lab value of a newborn with CF?
Neurons and RBCs
Meconium ileus and failure to thrive; negative serum immunoreactive trypsin
Dopamine
Cheilosis and corneal vascularization

33. How does glycerol from triacylglycerols in adipose tissue used for gluconeogenesis?
NADH - Acetyl CoA - ATP; NAD+ - ADP and Ca2+
Glutamate
Multiple bone fractures - blue sclerae - hearing loss (abdnormal middle ear bones!) - dental imperfections - retarded wound healing
Gets converted to glycerol 3 P Which is then converted to DHAP Which then can enter glycolyis

34. What does oligomycin do?
Cycloheximide
Aromatic; musty body odor
Block ATP synthase resulting in no ATP production and an increased proton gradient
pleiotropic

35. Because electron transport continues...
Structural (constitutively active)
CCA and chemically modified bases Where amino acid is covalently bonded
ATP depletion and highly aerobic tissues (heart and brain) are affected first; false - glucose infusion worsens
O2 consumption (last electron acceptor) gets consumed also

36. What form of Vitamin D is in plants? milk fortified with vit D? formed in sun exposed skin?
D2 (ergocalciferol) - D3 (cholecalciferol) - D3 (cholecalciferol)
Mental and growth retardation; seizures - fair skin - eczema - musty body odor; neurotoxic ketones and acids stimulate vomitting - can lead to pyloric stenosis
Indirect inhibtion of the Na+/Ca+2 exchange resulting in increased intracellular calcium Which increases cardiac contractility
COP I

37. What is uniparental disomy? give an example of a disease it can cause.
Fe2+; Fe3+
Cells that stay in G0 and cannot replicate - instead they regenerate from stem cells; neurons - skeletal and cardiac muscles - and RBCs
To increase serum Ca and Phosphate for ultimate bone deposition by: increase intestinal absorption of calcium and phosphate (also renal); and increase resorption of bone by stimulating macrophages (osteoclasts)
When child gets two chromosomes from one parent; imprinting dx

38. Is PRPP added later or first in pyrimidine synthesis?
Later
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
A cytoskeletal protein - apart of flagella - cilia - mitotic spindles and transport (in neurons for example)
tubulin dimer

39. What are causes of zinc deficiencies?
Alcoholism - RA - inflammatory disease - chronic diarrhea
Vit D levels
I; nucleolus
Folinic acid; does not require DHF

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

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41. What are labile cells? Which cells are labile?
Respiratory burst - P450 - and glutathione reductase
Cells that never got into G0 and divide rapidly with a short G1; bone marrow - gut epithelium - hair follicles - skin
Steroid - cholesterol FA synthesis; HMP shunt
Hurlers Syndromes;Iduronate sulfatase deficiency; alpha L iduronidase def; heparan sulfate - dermatan sulfate

42. Which amino acids are required during periods of growth?
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
They can enter G1 from G0 and replicate if they like; hepatocytes - lymphocytes - fibroblasts
Beta thalassemia
Arg and his

43. How is entry into cell cycle phases regulated?
All and Alzheimers
30S ribosomal RNA (small one)
By cyclin CDK complexes
allosteric regulation

44. What disease is associated with chromosome 13?
ATP depletion and highly aerobic tissues (heart and brain) are affected first; false - glucose infusion worsens
Wrinkles - acne and psoriasis topically - promyelocytic anemia - measles
Where there is insulin; low protein kinase A means dePhosphorylation of the complex Which results in increased PFK2 Which results in more fructose 2 -6 bp and thus more PFK1!!
Rb

45. polymerase chain reaction (PCR)- Name an application for it
Increased blood levels of ammonia - decreased BUN
Genotyping for mutant alleles (different sizes)
H1
1 for all three classes of RNA; Rifampin; TB

46. What disease manifestations result in Prader Willi syndrome? is the deletion a result from the father or mothers chromosome?
Mental retardation - obesity - short stature - hypogonadism - hypotonia - hyperphagia; father
Tryptophan; melatonin
Acetyl coA; gluconeogenesis
MT!

47. _______ can not get inside cells
Gets converted to glycerol 3 P Which is then converted to DHAP Which then can enter glycolyis
Hemolytic anemia due to heinz bodies Which are oxidized hemoglobin precipitating within the RBCs
to ions
fructose

48. What is the rate limiting step in fatty acid synthesis? What four things inhibit this rate limiting step? What stimulates it?
Lactose (glucose and galactose)
Mitochondrial inheritance
Pigmented iris hamartomas
Acetyl coA carboxylase converting Acetyl coA into malonyl coA; increased AMP - glucagon - epinephrine - palmitate; citrate

49. What are the clinical symptoms of NF2?
Bilateral acoustic schwannomas - juvenile cataracts
30S + 50S= 70S
Cleft palate - Abnormal faces - thymic aplasia (t cell def) - Cardiac defects - hypocalcemia (secondary to hypoparathyroid); 3rd and 4th branchial pouch aberrant development
Tryptophan; B6

50. Where are secreted proteins synthesized? Where are cellular/organellar proteins synthesized?
Succinyl coA
Mebendazole and thiabendazole
TCA cycle - FA oxidation - acetyl coA production; ox phos
RER; free ribosomes

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USMLE Step 1 Biochemistry

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