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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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This is a study tool. The 3 wrong answers for each question are randomly chosen from answers to other questions. So, you might find at times the answers obvious, but you will see it re-enforces your understanding as you take the test each time.

1. AR disease are usually...
adrenal medulla
enzymatic
Uses HMG coA reductase
Cardiomegaly - cyanosis - tachycardia - dyspnea - vomitting; 2-3 months

2. What is the treatment of hyperammonia?
Sickle cell disease - conversion of hydroophillic glutamic acid to hyrdophobic valine
In the reproductive tract and in the resp tract; infertility - bronchiectasis and recurrent sinusitis - associated with situs inversus
Limit protein in diet; give benzoate or phenylbutyrate Which bind amino acids and lead to excretion - lactulose (hydrogen ion binds to ammonia to make excretable ammonium) and neomycin (to kill bacteria that release ammonia from aa)
Alpha 1 -4 linkages; alpha 1 -6 linkages

3. What three enzyme def. can cause homocysteinura? What is their mode of inheritance?
Homocysteine methyltransferase - cystathionine reductase - decreased affinity of cystathionine reductase for pyridoxal phosphate; AR
Marfans - homocystinuria - MEN 2B
Osteomalacia is vit D def seen in adults and causes bowed legs and results in increased fractures
Arg and his

4. What aa becomes essential with phenylketonuria?
Tyrosine
Oral uridine administration; provides nucleosides and provides feedback inhibition
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
Consumption of bear liver - and overtreatment isoretinoin; constitutional symptoms and alopecia and skin changes

5. What ensures that during gluconeogenesis all the ATP wont be used in glycolysis as youre producing it?
Complex I - Complex II (lower energy)
Protein Kinase A inhibits pyruvate kinase also ATP and alanine have negative effects on pyruvate kinase
AD
Edwards

6. Only the tyrosinase one is...
2 NADH - 2 ATP - 2 pyruvate - 2H+ - 2H20
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)
Biotin; rate limiting step in fA synthesis - acetyl coA into malonyl coA
AR

7. What are the causes of folate def?
Because that is Where the products will be consumed; acetyl coA; TCA cycle of ketone body production
Increased orotic acid in urine - megaloblastic anemia - +/- hyperammonemia
Thick mucus plugs lungs recurrent pulmonary infections (pseudomonas and aureus) - chronic bronchitis - bronchiectasis (which can lead to cor pulmonale!); biofilms form on resp tract
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

8. What is the function of Vit D?
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)
Urea cycle enzyme deficiency (mc: ornithine transcarbamoylase); liver disease - alcoholic cirrhosis and reye's
DHF
B hydroxybutyrate - acetone - acetoacetate; beta hydroxybutyrate; beta hydroxybutyrate

9. What are the only substrates for gluconeogenesis? and What products are convertible to it?
To supply sufficient glucose to the brain and the RBCs and to preserve protein
Pyruvate (through OXA); lactate - alanine - malate (and all other TCA intermediates through OXA) - glycerol - odd chain fatty acids yield propionyl coA Which can be converted to succinyl coA and enter as TCA cycle
Shiga toxin and ricin (protein in castor beans)
Orotic acid in blood and urine (from pyrimidine synthesis) - decreased BUN - and symptoms of hyperammonemia

10. What are the clinical symptoms of Huntingtons disease? When does it present?
Depression - progressive dementia - choreiform movements; between the ages of 20 and 50
Either salvage by HGPRT and PRPP or converstion to xanthine and then to uric acid by xanthine oxidase
Carnitine shuttle
Prolonged Ab use

11. Which intermediate filament is found connective tissue? muscle? epithelial cells? neuroglia? neurons? What can intermediate filaments be used for?
Vimentin - desmin - cytokeratin - glial fibrillary acidic protein (GFAP) - neurofilaments; to differentiate between different cells
Cytoplasmic side; active
When water is pulled in; polyols (sugar alcohols); cataracts - retinopathy - peripheral neuropathy all seen in chronic hyperglycemia in diabetes
Complex I - Complex II (lower energy)

12. What is the major cause of B12 def? What is the clinical picture of this?
Hemorrhage with increased PT and PTT time but normal bleeding time
Cardiomegaly and systemic findings leading to early death; lysosomal alpha -1 -4- glucosidase (acid maltase)
Glycerol and propionyl coA; lactate and alanine
Pernicious anemia; older - mentally slow european woman who is anemic and icteric with a smooth shiny tongue and has broad based shuffling gait

13. 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?
Adrenal cells (steroid synthesis) - hepatocytes (p450 detox)
By turning it into fructose - but sorbitol first; glucose to sorbitol via aldose reductase and sorbitol to fructose sorbital dehydrogenase
H1
Dihydrofolate reductase; vitamin B12; homocysteine

14. What is imprinting?
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
Beta glucocerebrosidase; glucocerebroside
Inosine/IMP; adenosine deaminase
MT!

15. name the water soluble vitamins.
Macrocytic megaloblastic anemia; it is the most common vit def in the US; no neuro symptoms and no methylmalonyl coA in the urine
Vitamin B1 (thiamine) - Vitamin B2 (riboflacin) - Vitamin B3 (niacin) - Vitamin B5 (pantothenate) - Vitamin B6 (pyridoxine) - Vitamin B7 (Biotin) - Vitamin B12 (cobalamin) - folate - Vit. C
Phosphofructokinase 1; fructose 1 phosphate to fructose 1 -6 bisphosphate; enhancers: AMP - fructose 2 -6 Bisphosphate; downregulators: citrate - ATP
Mental retardation - obesity - short stature - hypogonadism - hypotonia - hyperphagia; father

16. The mode of inheritance for both NF disease is...
S. aureus - Aspergillus
VHL (Von Hippa Lindau)
Both AD
Hypophosphatemia rickets and Alports

17. What is the inheritance pattern of hereditary spherocytosis?
AD
A lysosomal storage disease; mannose 6 phosphate can not be added to lysosomal proteins
Some viruses
All clathrin

18. What are the findings in ornthinie transcarbamoylase?
In the charging reaction! for this reason aminoacyl tRNA synthetase double checks
Mitochondrial inheritance
Mitochondria - archaebacteria - mycoplasma - and some yeasts
Orotic acid in blood and urine (from pyrimidine synthesis) - decreased BUN - and symptoms of hyperammonemia

19. What is the rate determining enzyme in TCA cycle?
1 for all three classes of RNA; Rifampin; TB
Decrease AFP - increased BCHG - decreased estriol - increased inhibin A
Aldose reductase in the lens converts built up galactose to galactitol Which is osmotically active
Isocitrate dehydrogenase

20. What does FISH stand for? What is it used for? Give an example.
Uracil; thymine
Flourescent DNA/RNA In SITU hybridization; for visualization of anomalies (like microdeletions) that are too small to be seen on karyotype; Prader - Willi syndrome
1 day; adipose release of FFA - hepatic gluconeogenesis from peripheral tissue lactate and alanine and from adipose tissue glycerol and propionyl coA
Mitochondria - archaebacteria - mycoplasma - and some yeasts

21. What is used in a marathon?
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
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
Glycogen and FFA oxidation - glucose conserved for final sprinting!
Alpha amanitin; mushroom caps

22. What else decreases production of oxaloacetate in alcoholism other than decreased pyruvate?
Oxaloacetate gets shifted to malate d/t excess NADH
Respiratory - seminal fluid - pancreatic - gallbladder; very thick without NaCl to pull water
Glutamate; glutamate decarboxylase; B6; gamma aminobutyrate
Neurologic lesions and cardiomyopathies (ox phos!)

23. What kind of bugs are CGD patients screwed with? why do they call it chronic granulamotous disease?
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
tumor suppressor genes (p53 - Rb - BAX)
ATP
Dark connective tissue - brown pigmented sclera - debilitating athralgias; homogentistic acid builds up and can be toxic to cartilage

24. Odd chain fatty acids yield one proprionyl coA which can enter the TCA cycle as...
Telomerase; an RNA polymerase that uses a RNA template to add DNA to the end of shortening 3' ends
Cartilage (including hyaline) - vitreous body - nucleus pulposus
succinyl coA (and then to OXA)
Branching increases the rate of synthesis and breakdown of glycogen by having more sites to add on to and break off

25. 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?
DOPA
alk phos
Endonucleases are specific and break specific linking phosphodiester bonds - exonucleases are not sequence specific and just remove the nucleoside
Stored ATP - creatine phosphate - anaerobic glycolysis; as distances increase - ATP is obtained from additional resources; above plus ox phos

26. What does adipose tissue contribute for hepatic gluconeogenesis during starvation? muscle?
Renal calculi of uric acid (replaces uric acid in excretion)
By epoxide reductase in the liver; coumadin; intestinal flora and leafy vegetables
Tetrahydrofolates
Glycerol and propionyl coA; lactate and alanine

27. What is the fate of glucose from glycogenolysis?
Flourescent DNA/RNA In SITU hybridization; for visualization of anomalies (like microdeletions) that are too small to be seen on karyotype; Prader - Willi syndrome
Malabsorption problems (CF and sprue) and mineral oil intake
Gauchers; hepatosplenomegaly - aseptic necrosis of the femur - bone crises - Gauchers cells - crumpled tissue paper looking (fibrillar appearing macrophages in liver - spleen - b.m.)
Glucosekinase to glucose 6 P

28. In x linked - in males - prevalence is...
Lactating mammary glands - adrenal cortex (sites of fatty acid or steroid synthesis) - RBCS (for glutathione reduction for FR injury)
Chorea and spastic paresis
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
= q

29. What are the clinical findings in patau syndrome?
Synthesis in liver and then hydrolysis of chylomicrons and VLDLs by capillary lipoprotein lipase in peripheral tissue; adipose tissue by lipases
Mental retardation - rocker bottom feet - micropthalmia - microcephaly - cleft lip/palate - holoprosoncephaly - polydactyly
NADPH- reductive biosynthesis!
Limit dextrans

30. Findings for McArdle's disease? deficient enzyme?
The products of FA oxidation (Acetyl CoA and NADH) phosphorylate pyruvate deyhdrogenase Which inactivates it
Tyrosine
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
Both AD

31. What is the effect of defiiency of VIt E?
The products of FA oxidation (Acetyl CoA and NADH) phosphorylate pyruvate deyhdrogenase Which inactivates it
3'--> 5'; prokaryote only
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
pleiotropic

32. Which glycogen storage disease causes hypertriglyceridemia?
By addition of mannose 6 phosphate to proteins in the Golgi
Von gierkes
1) if it is stable/labile or permanent 2) intact BM and 3) intact ECM
Abs against type IV collagen; pulmonary and glomerular capillaries

33. What is the mode of inheritance of Huntingtons?
Glycolysis and aerobic respiration
2-3 days after birth because maternal enzyme during fetal like might still be there
AD (trinucleotide expansion is not a mode of inheritance)
Block ATP synthase resulting in no ATP production and an increased proton gradient

34. What are the findings in AR Hurlers?
By giving high amounts of pyridoxal phosphate in the diet
Developmental delay - gargoylism - airway obstruction (d/t short neck) - corneal clouding - CAD - hepatosplenomegaly
A 16C FA! the product in FA synthesis; saturated; no double bonds
Biotin; rate limiting step in fA synthesis - acetyl coA into malonyl coA

35. Do introns or exons contain the genetic information?
Antineoplastic agents; alkylate DNA!; cyclophosphamide and nitrosurea (aka lomustrine - a nitrosource)
Exons
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
Codons may encode same amino acid since there are only ~20 aa

36. What disease is associated with chromosome 13?
IMP
Hepatic glycogenolysis (mostly) - also hepatic gluconeogenesis - adipose release of FFA (minor)
A topoisomerase II inhibitor; because cancer cells use II more so than healthy cells
Rb

37. Which water soluble vitamins get stored in the liver with the fat soluble vitamins? Which has a greater pool of storage in the liver? Where do all the other water soluble vitamins go?
Glutamate; glutamate decarboxylase; B6; gamma aminobutyrate
B12 and folate; B12; washed out
glutton
Can inducibly manipulate genes at specific developmental points

38. 'Initiation' of protein tranlsation is activated by ATP hydrolysis. T/F?
Decreased phenylalanine - increase tyrosine
False; GTP hydrolysis
cell cycle events
Intestine - renal - bone

39. What cofactor is necessary for conversion of dopamine to NE? NE to epinephrine?
SAM; Vit C
Robertsonian translocations
Flourescent DNA/RNA In SITU hybridization; for visualization of anomalies (like microdeletions) that are too small to be seen on karyotype; Prader - Willi syndrome
Pyruvate (through OXA); lactate - alanine - malate (and all other TCA intermediates through OXA) - glycerol - odd chain fatty acids yield propionyl coA Which can be converted to succinyl coA and enter as TCA cycle

40. What converts dopamine to norepinephrine? using What cofactor?
Dopamine beta hydroxylase; vit C
Cafe au lait spots - optic pathway gliomas - lisch nodules - scoliosis - neural tumors
Downs
Cytosine to uracil

41. Other than thymidylate synthase - What else is required to convert dUMP to dTMP?
Citrate is positive on it - AMP and fructose 2 -6 bisphosphate is negative on it; gluconeogenesis
Endonucleases are specific and break specific linking phosphodiester bonds - exonucleases are not sequence specific and just remove the nucleoside
N5N10 methylene THF
Tetrahydrofolates

42. What three steps are included in RNA processing? Where does it occur?

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43. in What disease are there antibodies to snRNPs?
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
Damaged 'ubiquinated' cytokeratin intermediate filaments build up in hepatocytes - damaged neurofilaments in idiopathic parkinsons disease build up to form Lewy bodies - Neurofibrillary tangles are build up of neurofilaments in alzheimers
Lupus
When child gets two chromosomes from one parent; imprinting dx

44. What is given to prevent side effects When methotrexate is given? why does this work?
S adenosyl homocysteine; homocysteine transferase; B12 and THF; M- THF
Folinic acid; does not require DHF
1 for all three classes of RNA; Rifampin; TB
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

45. What converts dihydrobioterin back to tetrahydrobioterin factor? What energy carrier is used? in What reactions is this cofactor used?
Decreased substrate for gluconeogenesis (pyruvate to lactate and not oxaloacetate); and pyruvate doing only lactic acidosis and not glycolysis
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
Dihydropterin reductase; NADP+; phenylalanine to tyrosine and tyrosine to DOP
Phosphoenolpyruvate; pyruvate kinase; fructose 1 - 6 BP; ATP - alanine

46. What aa is the precursor for creatinine?
Fructokinase; benign - fructose in the blood and urine
Arginne
Ornithine transcarbomyalse; orotic aciduria; X linked recessive; autosomal recessive
By epoxide reductase in the liver; coumadin; intestinal flora and leafy vegetables

47. What is the exception to degeneracy in the genetic code?
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
only L form
Methionine is only encoded by AUG
fat

48. How do you treat homocystinuria due to decreased affinity of cystathionine reductase to B6?
ATP and biotin
By giving high amounts of pyridoxal phosphate in the diet
Glutamine - aspartate - glycine; aspartate
RBCs - renal medulla - leukocytes - lens - testes - cornea

49. In What syndrome is HGPRT deficient? What is the inheritance pattern?
Arginine
Lesch Nyhan syndrome; X linked recessive
Phenylethanolamine N- methyl transferase; SAM
Actin/myosin - MT - intermediate filaments

50. Which two lysosomal storage disease are XR and not AR?
Liver; berry aneurysms (d/t hypertension!) - and mitral valve prolapse
Fabrys Disease and Hunters Syndrome
False; GTP hydrolysis
Water soluble vitamin stored in liver; converted to THF by dihydrofolate reductase and is a coenzyme in 1 methyl transfers - impt for DNA synth