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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.
Don't refresh. All questions and answers are randomly picked and ordered every time you load a test.

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. What is the pathogenesis of skurvy? name some clinical symptoms.
Hemolytic anemia; jaundice in newborn if mom gets too much vit K
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
Cheilosis and corneal vascularization
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

2. 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?
Dilated capillaries and veins; skin - mucous membranes - GI - resp - urinary tracts; rupture and can cause epistaxis - hemorrhage - GI bleeding
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!!
B12 and folate; B12; washed out
Carnitine acyltransferase I

3. other than succinate dehydrogenase What other enzymes are found in the inner mit membrane?
Because that is Where the products will be consumed; acetyl coA; TCA cycle of ketone body production
ATP synthase - ATP- ADP translocase - ETC
7 dehydrocholesterol
Chediak Higashi Syndrome; decreased phagocytosis results in recurrent pyogenic infections - partial albinism - and peripheral neuropathy

4. 'Initiation' of protein tranlsation is activated by ATP hydrolysis. T/F?
False; GTP hydrolysis
Karyotyping
A cytoskeletal protein - apart of flagella - cilia - mitotic spindles and transport (in neurons for example)
Smoking (FR takes up all the antioxidants); diets lacking fruit

5. What are the clinical symptoms that result from I cell disease?
Coarse facial features - psychomotor retardation - clouded corneas - restricted joint movement - high plasma levels of lysosomal enzymes
NADPH- reductive biosynthesis!
Glucosekinase to glucose 6 P
plasma membrane

6. What is most often mutated in xeroderma pigmentosum? What is the inheritance pattern? What are the clinical symptoms?
10 - 9 - 7 - 2; protein C and S
Renal failure - decreased sun - fat malabsorption - chronic liver disease - p450 induction - hypoparathyroidism (activates 1 alpha hyrdoxylase)
Using serine (which turns into glycine after reaction)
UV specific endonucleases for excision repair; AR; dry skin - photosensitivity - poikiloderma - susceptibility to skin cancers

7. What is the degradation product of Norepinephrine?
tumor suppressor genes (p53 - Rb - BAX)
XR - iduronate sulfatase
VMA
glycogenolysis

8. What is adenosine/AMP converted to? through What enzyme?
Inosine/IMP; adenosine deaminase
So that excess glucose can be stored (phosphorylated glucose cant leave) in the liver and saved as a buffer
No protein!
Also have fair skin - blond hair - blue eyes - eczema

9. list three uncoupling agents
TCA cycle - FA oxidation - acetyl coA production; ox phos
2 -4 dinitrophenol - aspirin - and thermogenin (in brown fat)
Cells that stay in G0 and cannot replicate - instead they regenerate from stem cells; neurons - skeletal and cardiac muscles - and RBCs
30S + 50S= 70S

10. What are the resp. clinical symptoms of CF?
An AP (apyrimidinic) endonuclease to cut the DNA - exonuclease then removes and then filled and sealed by enzymes
Galactokinase; galactose appears in blood and urine - sometimes infantile cataracts may present as failure to track objects or lack of a social smile
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!!
Thick mucus plugs lungs recurrent pulmonary infections (pseudomonas and aureus) - chronic bronchitis - bronchiectasis (which can lead to cor pulmonale!); biofilms form on resp tract

11. What is elastin? How is it different from collagen? Where is it found? How is it broken down?
TFs
Degeneration of retinal ganglion cells and axons; acute loss of central vision; mit. inheritance
A fibrous protein (ie produced by fibroblasts); it has lysine and proline but nonglycosylated; arteries - lungs - elastic ligaments - vocal cords - ligamenta flava (connect vertebrae); broken down by elastase Which is inhibited by alpha 1 antitrypsin
Coenzyme A and lipoamides

12. What tissues contain aldose reductase? What does this enzyme do to what?
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
Tyrosine; phenylaline
Lens and neural tissue; converts galactose to osmotically active galactitol
Avidin in raw egg whites prevents their reabsorption; also antibiotic use; dermatitis - alopecia - glossitis - lactic acidosis - enteritis

13. What can cause an excees of vit A? What are the symptoms?
Williams
can be d/t lyonization
Consumption of bear liver - and overtreatment isoretinoin; constitutional symptoms and alopecia and skin changes
Alpha galactosidase A; ceramide trihexoside

14. What aa is most accumulated in hartnup disease? What vitamin becomes deficient?
Cartilage (including hyaline) - vitreous body - nucleus pulposus
Mental retardation - obesity - short stature - hypogonadism - hypotonia - hyperphagia; father
Tryptophan; niacin
By giving high amounts of pyridoxal phosphate in the diet

15. Daughters of Xlinked recessive diseased fathers are...
They can enter G1 from G0 and replicate if they like; hepatocytes - lymphocytes - fibroblasts
Chloramphenicol
all carriers
TPP (from B1- pyrophosphate) - FAD (B2) - NADH2 (B3) - CoA (B5 - panothenate) - Lipoic Acid

16. What is the pathogenesis of fragile x syndrome?
A trinucleotide repeat disorder results in faulty methylation and thus expression of the FMR1 gene on the X chromosome (x linked recessive mode of inheritance)
are AR
Osteomalacia is vit D def seen in adults and causes bowed legs and results in increased fractures
Tyrosinase deficiency; defective tyrosine transporters; lack of migration of neural crest cells; skin cancer

17. What is the main aa composition of collagen? What other 2 aa are common?
Ornithine transcarbomyalse; orotic aciduria; X linked recessive; autosomal recessive
Glucose 6 phosphate dehydrogenase
Glycine; proline and lysine
Mitochondria - archaebacteria - mycoplasma - and some yeasts

18. What type of vitamin is Vit D? Where do we get it? in What form?
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)
Mental retardation - rocker bottom feet - micrognathia (small jaw) - low set ears - clenched hands with polydactyly - congenital heart disease
AAUAAA; Poly A polymerase; no
Lack of lactase (brush border enzyme) so can not break down lactose (a dissaccharide of galactose and glucose); African Americans and Asians

19. What is uniparental disomy? give an example of a disease it can cause.
When child gets two chromosomes from one parent; imprinting dx
Gets converted to pyruvate via the Cori cycle in the liver
teratogenic
Niacin treatment for hyperlipidemia; facial flushing

20. What three things contribute to fatty change in the liver in alcoholism?
Niemann pick and Tay Sachs; hepatosplenomegaly or not
Because that is Where the products will be consumed; acetyl coA; TCA cycle of ketone body production
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
Respiratory - seminal fluid - pancreatic - gallbladder; very thick without NaCl to pull water

21. What form of Vitamin D is in plants? milk fortified with vit D? formed in sun exposed skin?
D2 (ergocalciferol) - D3 (cholecalciferol) - D3 (cholecalciferol)
The active form of Vit D (1 -25 (OH)2 D3)
(cofactor for 100+ enzymes!) needed for zinc fingers transcription motif - collagen synthesis - alcohol metabolism
AD

22. What are three ways to poison Ox phos?
Enriched whole grain cereals and other things; thiamine; malnutrition (nonenriched white processed bread and rice) and alcohol second to malnutrition
Electron transport inhibitors - ATPase inhibitors - Uncoupling agents (increase permeability of the membrane - loss of hydrogen gradient)
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
N linked oligosaccharides

23. What are the two most common aminotransferase and What amino acids do they turn to What glycolytic intermediates?
Blood group types
Mental retardation - rocker bottom feet - micropthalmia - microcephaly - cleft lip/palate - holoprosoncephaly - polydactyly
Double strand breaks; nonhomologous end joining
Alanine aminotransferase (ALT) and aspartate aminotransferase (AST); alanine to pyruvate and aspartate to oxaxloacetate

24. How is Vit K activated? What drug blocks this? What is the source of Vit K?
By epoxide reductase in the liver; coumadin; intestinal flora and leafy vegetables
ATP synthase - ATP- ADP translocase - ETC
Bacteria produce lactate acid and gases from lactose and produce acidic diarrhea and bloating
A ketone

25. What two compounds provide the two N's of urea? What else is in urea other than two NH2's?
Glutamate and aspartate; CO2 provides an ketone
NAD+; through ETC you produce NAD+; through production of Lactate from pyruvate you create lactate
cell cycle events
Increased CPK and muscle biopsy

26. How do you treat niacin deficiency do to corn based diet?
Lime and CaCO3 to displace the niacin
Nacetyl cysteine; breaks disulfide bridges of mucus plugs
homocysteine transferase
30S + 50S= 70S

27. Cysteine is one of two _____ connected by disulfide bond
Arg - lys - his; positive except His has no charge at body pH
dimers
high cholesterol
Pulmonary - GI - reproductive (infertility in male due to absence of vas deferens and seminal fluid thickness)

28. What does severe deficiency of B3 lead to? What are the causes?
Catalyze gamma carboxylation of glutamic acid residues on coag factors so they can bind to Ca complex
to ions
Schwann cells - lens - retina - kidneys because they only have aldose reductase and no sorbital dehydrogenase
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)

29. Achondroplasia is associated with advanced ________ age
Steroid - cholesterol FA synthesis; HMP shunt
paternal
Via fructose 1 P by fructokinase in the liver; as glucose 1 by being GALT- ed ;)
Phenylalanine hydroxylase; tetrahydrobiopterin factor

30. What is the result of lactase deficiency? in What nationalities is it seen the most?
Lack of lactase (brush border enzyme) so can not break down lactose (a dissaccharide of galactose and glucose); African Americans and Asians
Glutathione peroxidase (catalase) reduces H202 to H20 with glutathione; oxidized glutathione gets reduced by glutathione reductase with NADPH; NAP+ is regenerated to NADPH with Glucose 6 phosphate dehydrogenase; because H2O2 from the resp burts in th
long saturated fatty acids
Meiotic nondisjunction in mothers gametes during the first meiosis

31. What is the mode of inheritance of Marfans?
Orotic acid in blood and urine (from pyrimidine synthesis) - decreased BUN - and symptoms of hyperammonemia
Pyruvate dehydrogenase; ATP - NADH - acetyl CoA
UV nonionizing radiation causing thymidine dimers
AD

32. What is the source of Biotin?
Karyotyping
Bacterial synthesis in the intestine
Dihydrofolate reductase; vitamin B12; homocysteine
Dynein - retrograde - kinesin - anterograde

33. What causes hypoglycemia in alcoholism?
Decreased substrate for gluconeogenesis (pyruvate to lactate and not oxaloacetate); and pyruvate doing only lactic acidosis and not glycolysis
In ability to reabsorb phosphate from renal tubular cells results in osteomalacia due to defective bone mineralization; no
Fat malabsorption (CF) and abetalipoproteinemia (damage takes up all antioxidants)
Myoclonus epilepsy

34. Name two antineoplastic drugs that work by inhibiting the mitotic spindle by MT blocking.
pleiotropic
Cortisol; inhibits insulin action
Vincrstine (vinca alkaloids) and paclitaxel
Glucagon - insulin and epinephrine

35. Hexokinase has a HIGH affinity (low Km) and...
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
Silent - misssense - nonsense
low Vmax
Tryptophan; niacin

36. If a cell has a hyperchromatic or condensed nucleus is it undergoing transcription etc?
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!!
When water is pulled in; polyols (sugar alcohols); cataracts - retinopathy - peripheral neuropathy all seen in chronic hyperglycemia in diabetes
No
ATP depletion and highly aerobic tissues (heart and brain) are affected first; false - glucose infusion worsens

37. What causes cataracts in a galactose metabolism deficiency?
Alpha ketoglutarate; no TCA cycle intermediates
AST (mit damage) - and GGT (d/t SER hyperplasia d/t induction of p450 generation)
Alcoholism - RA - inflammatory disease - chronic diarrhea
Aldose reductase in the lens converts built up galactose to galactitol Which is osmotically active

38. What are chromosomes? during What part of the cell cycle are they found? in What pathologic state would you see a lot of chromosomes?
Highly condensed forms of DNA; mitosis; neoplasia - highly dividing and thus hyperchromatic
Night blindness - dry skin
X linked recessive; deletion of the dystrophin gene (connects actin to the membrane glycoprotein) due to frame shift mutation - lack of dystrophin results in muscle breakdown Which gets replaced by fibrofatty tissue and collagen
Cytosine to uracil

39. What is the precursor in the skin that the sun turns to D3?
Phospholipids - sphingolipids - glycolipids
Liver mit and liver cytosol
Thinned cornea from lack of connective tissue so see underlying choroidal lens
7 dehydrocholesterol

40. What is hypophosphatemia rickets? Can it be treated with Vit D?
5' end; 3' end
Acetyl coA activated pyruvate carboxylase in gluconeogenesis
In ability to reabsorb phosphate from renal tubular cells results in osteomalacia due to defective bone mineralization; no
Hungtington

41. Where does NADH2 enter ETC? FADH2
Homocysteine; homocysteine methyltransferase with B12 and tetrahydrofolate
Complex I - Complex II (lower energy)
Dihydrofolate reductase; vitamin B12; homocysteine
G6PD dehydrogenase; transketolases

42. What disease is associated with chromosome 21?

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43. why does a deficiency in ornithine transcarbamoylase (urea cycle enzyme) result in an increased production of orotic acid - the pyrimidine precursor?
Pulmonary - GI - reproductive (infertility in male due to absence of vas deferens and seminal fluid thickness)
Hemolytic anemia; jaundice in newborn if mom gets too much vit K
Because carbamaoyl phosphate is involved in both urea cycle and pyrimidine synthesis
It contains the peptidyltransferase activity (for elongation of protein

44. What are the findings in maple syrup disease? What should be tried to be given as treatment?
Alpha ketoglutarate; no TCA cycle intermediates
Flourescent DNA/RNA In SITU hybridization; for visualization of anomalies (like microdeletions) that are too small to be seen on karyotype; Prader - Willi syndrome
CNS defects - mental retardation - death - feeding difficulties - vomiting - hypoglycemia; thiamine
Preprocollagen - procollagen - tropocollagen - collagen

45. Which intermediate is involved in both pyrimidine synthesis and the urea cycle?
Carbamoyl phosphate
Tryptophan
Fruits and vegetables
Protein Kinase A inhibits pyruvate kinase also ATP and alanine have negative effects on pyruvate kinase

46. What are permanent cells? Which cells are permanent?
calorie deficient
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)
Cells that stay in G0 and cannot replicate - instead they regenerate from stem cells; neurons - skeletal and cardiac muscles - and RBCs
Calcium and calmodulin activate glycogen phosphorylase kinase

47. Give an example of codominance.
high cholesterol
SAM; Vit C
Acetyl coA activated pyruvate carboxylase in gluconeogenesis
Blood group types

48. What is a nissl body?
RER in nucleus (synthesize NTs)
Carbidopa
Cardiomegaly and systemic findings leading to early death; lysosomal alpha -1 -4- glucosidase (acid maltase)
Glycolysis and aerobic respiration

49. other than Acetyl coA inhibiting pyruvate dehydrogenase complex - how else is it ensured that gluconeogenesis is occuring during FA oxidation and not glycolysis?
Degeneration of retinal ganglion cells and axons; acute loss of central vision; mit. inheritance
linked recessive
Acetyl coA activated pyruvate carboxylase in gluconeogenesis
Lyonization; nondisjunction in mitotic divisions during embryonic period

50. What does oligomycin do?
So that excess glucose can be stored (phosphorylated glucose cant leave) in the liver and saved as a buffer
Block ATP synthase resulting in no ATP production and an increased proton gradient
By giving high amounts of pyridoxal phosphate in the diet
Tryptophan; melatonin