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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. What are the resp. clinical symptoms of CF?
Thinned cornea from lack of connective tissue so see underlying choroidal lens
Thick mucus plugs lungs recurrent pulmonary infections (pseudomonas and aureus) - chronic bronchitis - bronchiectasis (which can lead to cor pulmonale!); biofilms form on resp tract
Peripheral neuropathies of hands/feet; angiokeratomas between umbilicus and knees; cardiovascular/renal disease
Acetyl CoA carboxylase

2. What are the clinical symptoms of Down syndrome?
enzymatic
Cardiomegaly - cyanosis - tachycardia - dyspnea - vomitting; 2-3 months
Mental retardation - flat faces - simean crease - epicanthal folds - ASD - duodenal atresia causing Hirshsprungs disease - redundant skin at the nape of neck - slanted palpebral fissures - hypotonia
Less; more

3. Which glycogen storage disease results cardiomegaly?
Pompes
Lys and arg
Mental retardation - flat faces - simean crease - epicanthal folds - ASD - duodenal atresia causing Hirshsprungs disease - redundant skin at the nape of neck - slanted palpebral fissures - hypotonia
Schwann cells - lens - retina - kidneys because they only have aldose reductase and no sorbital dehydrogenase

4. Give examples of the anabolic processes that NADPH participates in. From What process is NADPH produced?
Steroid - cholesterol FA synthesis; HMP shunt
Citrate is positive on it - AMP and fructose 2 -6 bisphosphate is negative on it; gluconeogenesis
In the charging reaction! for this reason aminoacyl tRNA synthetase double checks
Cardiomegaly - cyanosis - tachycardia - dyspnea - vomitting; 2-3 months

5. What enzyme is deficient in Essential fructosuria? What are the symptoms?
Succinyl coA
Fructokinase; benign - fructose in the blood and urine
Mit encephalopathy - lactic acidosis - stroke like episodes
A lysosomal storage disease; mannose 6 phosphate can not be added to lysosomal proteins

6. In protein translation - What is the 'A' site for? 'P' site? 'E' site? What is the exception to this?

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7. From What aa does GABA come from? with What enzyme? and What vit cofactor? What does GABA stand for?
A cytoskeletal protein - apart of flagella - cilia - mitotic spindles and transport (in neurons for example)
Glutamate; glutamate decarboxylase; B6; gamma aminobutyrate
Osteomalacia is vit D def seen in adults and causes bowed legs and results in increased fractures
G6PD; provides malarial resistance; X linked recessive

8. Which antifungal acts on MT?
Hepatic glycogenolysis (mostly) - also hepatic gluconeogenesis - adipose release of FFA (minor)
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
Alpha amanitin; mushroom caps
Griseofulvin

9. What is the difference b/w beckers and duchennes muscular dystrophy?
AR
Downs (21) - Edwards (18) - Pataus (13); also in increasing severity
Acetyl coA activated pyruvate carboxylase in gluconeogenesis
Beckers is less severe (instead of deletion is either defect or deficiency) with later presentation

10. What is the mc chromosomal disorder?
Uric acidemia; HGPRT; purine salvage
Downs
homocysteine transferase
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

11. After N5N10 methylene THF donates a methyl for conversion of dUMP to dTMP What is it converted to?
2-3 days after birth because maternal enzyme during fetal like might still be there
DHF
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
Arg

12. What carries aldehydes as energry?
NF1
As BUN (blood urea nitrogen)
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
TPP (thiamine pyrophosphate)

13. What are the two 22q11 deletion syndromes? What is their pathogenesis? What is the difference between them?
TRNA wobble
Hypophosphatemia rickets and Alports
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
Citrate shuttle in the inner mitochondrial membrane transports acetyl coA into the cytoplasm; liver; liver and adipose tissue

14. What are three ketone bodies? Which is not detected in urine? Which do you see most in alcoholic ketoacidosis?
5' end; 3' end
Aldose reductase in the lens converts built up galactose to galactitol Which is osmotically active
B hydroxybutyrate - acetone - acetoacetate; beta hydroxybutyrate; beta hydroxybutyrate
Sulfonamides - primaquine - dapsone - antiTB drugs - fava beans - LDL; RBCs

15. What is the mode of inheritance for the multiple endocrine neoplasias?
Later
Ortinithine transcarbamoylase - orotic acid phosphoribosyltransferase - and orotidine 5 phosphate decarboxylase; urea cycle; last two are involved in conversion of orotic acid to UMP (adding PRPP to orotic acid)
Cytoplasmic side; active
AD

16. P450 induction decreases ___________ by increased metabolism
Vit D levels
Oxygen!
Dactinomycin - doxorubicin
IMP

17. What is DNA gyrase?
A topoisomerase II
Autosomal Dominant; NF1 and mit. diseases (variable expression) - ?
fat
When different combination of exons get combined together; in Beta thallasemia

18. What is the first step in glycolysis?
Glucose 6 phosphate dehydrogenase
Phosphorylation of glucose
Meiotic nondisjunction in mothers gametes during the first meiosis
CCA and chemically modified bases Where amino acid is covalently bonded

19. What type of proteins have AD mode of inheritance?
Leucine and lysine
Type IV collagen defect; X linked recessive; hereditary glomerulonephritis - deafness - ocular disturbances
Structural (constitutively active)
Decrease intake of fructose and sucrose (fructose and glucose)

20. Glucokinase has a LOW affinity (high Km) and...
Tay Sachs - Niemann Picks - Gauchers
high Vmax
Vitamin B1 (thiamine) - Vitamin B2 (riboflacin) - Vitamin B3 (niacin) - Vitamin B5 (pantothenate) - Vitamin B6 (pyridoxine) - Vitamin B7 (Biotin) - Vitamin B12 (cobalamin) - folate - Vit. C
Citrate shuttle in the inner mitochondrial membrane transports acetyl coA into the cytoplasm; liver; liver and adipose tissue

21. What does a deficiency in homogentistic acid oxidase result in? What is this also known as?
Gets converted to glycerol 3 P Which is then converted to DHAP Which then can enter glycolyis
Alkaptonuria; ochronosis
RBCs - renal medulla - leukocytes - lens - testes - cornea
Down's Syndrome

22. What can cause hypervitaminosis D?
Progressive neurodegeneration (spasticity d/t UMN disease) - NO hepatosplenomegaly - cherry red spot on macula - lysosomes with onion skin - muscle weakness
usually present late
Iatrogenic - granulomatous disease (sarcoidosis d/t epitheloid macrophage release of calcitriol); hypercalcemia - hypercalciuria (renal calculi!) - loss of appetite - stupor
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

23. What explains the excess phenylketones in the urine in phenylketonuria?
Assist in spontaneous refolding of proteins
Preprocollagen - procollagen - tropocollagen - collagen
Phenylalanine build up
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

24. What disease is associated with chromosome 17?
Glucokinase
NF1
Alpha galactosidase A; ceramide trihexoside
ATP synthase - ATP- ADP translocase - ETC

25. What is deficient in someone with hereditary hypoketotic hypoglycemia?
fastest sugar
Indirect inhibtion of the Na+/Ca+2 exchange resulting in increased intracellular calcium Which increases cardiac contractility
IMP
Carnitine shuttle

26. What is the other name of Vitamin B6? What is its function?
Pyridoxine; converted to pyridoxal phsophate used in transamination reaction - decarboxylations - heme synthesis - niacin synthesis from trp - glycogen phosphorylase - cystathionine synthesis - NT synthesis
By epoxide reductase in the liver; coumadin; intestinal flora and leafy vegetables
Isoleucine - valine - leucine
DOPA

27. What is the rate limiting step in glycolysis? What does it do? who activates it? inhibits it?
Mcardles disease
Acidic
Pataus
Phosphofructokinase 1; fructose 1 phosphate to fructose 1 -6 bisphosphate; enhancers: AMP - fructose 2 -6 Bisphosphate; downregulators: citrate - ATP

28. What type of enyzmes are used in base excision repair?
VHL (Von Hippa Lindau)
Glycosylases (only remove the base - cut the bond between ribose and base)
ATP and biotin
Succinate hydrogenase; matrix

29. What is elastin? How is it different from collagen? Where is it found? How is it broken down?
Convulsions - hyperirritiability - peripheral neuropathy - sideroblastic anemias; alcoholics and INH and fortified goats milk consumption
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
A methyl
Succinate hydrogenase; matrix

30. Which TCA cycle enzyme is found in the inner mit membrane? Where are the rest found?
Galactose gets converted to galactose 1 Phosphate by galactokinase Which then gets turned into glucose 1 phosphate by Galactose 1 Uridyl Transferase (GALT) Which also produces UDP Gal Which is used for lactose production in the breast; the fate of gl
Succinate hydrogenase; matrix
Phospholipids - sphingolipids - glycolipids
No

31. What negatively regulates dopamine to norepinephrine?
Central and peripheral demyelination with ataxia and dementia
Carbidopa
Dilated capillaries and veins; skin - mucous membranes - GI - resp - urinary tracts; rupture and can cause epistaxis - hemorrhage - GI bleeding
Aromatic; musty body odor

32. What is the first step of collagen synthesis? 2nd step? 3rd step? 4th step? Where do all these steps occur?
ATP
Formation of the alpha chains out of glycine - lysine - proline in the RER to make preprocollagen; hydroxylation of the proline and lysine residues in the RER; glycosylation of the lysine residues to make procollagen Which is triple helix of alpha ch
Lysine and arginine; basic
hexokinase - glucokinase (its a glutton)

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
2 oxidative and nonoxidative; cytoplasm (both); none (none produced either - you are simply switching carriers of energy- from glucose 6 phosphate to NADPH)
Inosine/IMP; adenosine deaminase
Neurologic defects - myopathy - lactic acidosis; giving ONLY ketogenic nutrient high fat content and aa (lysine and leucine)

34. What is the source of Vitamin B1? What is its other name? What causes a deficiency in B1?
kidney
Retinol; Beta carotenes (dark and yellow vegetables)
Enriched whole grain cereals and other things; thiamine; malnutrition (nonenriched white processed bread and rice) and alcohol second to malnutrition
Some viruses

35. What is the exception to degeneracy in the genetic code?
DHF
Glycolysis and aerobic respiration
Methionine is only encoded by AUG
By epoxide reductase in the liver; coumadin; intestinal flora and leafy vegetables

36. What is the inheritance of Von Hippel Lindau disease?
NADPH oxidase - superoxide dismutase - myeloperoxidase; in the phagolysosome; HOCl (bleach); kills!
both liver and muscle (unlike glucagon)
AD
Because carbamaoyl phosphate is involved in both urea cycle and pyrimidine synthesis

37. What enzyme is deficient in galactokinase deficiency? What are the symptoms?
Galactokinase; galactose appears in blood and urine - sometimes infantile cataracts may present as failure to track objects or lack of a social smile
Macroorchitism - big ears - big jaw - mental retardation (2nd mcc!) - mitral valve prolapse - long face
Neimann pick and tay sachs
Double strand breaks; nonhomologous end joining

38. The __________ adds O linked oligosaccharides to serine and threonine residues on proteins
Pompes
Phosphorylation of glucose
golgi apparatus
Arg - lys - his; positive except His has no charge at body pH

39. What is the pathogenesis of fragile x syndrome?
Do an RT PCR (reverse transcriptase)- RT the RNA first - then PCR the DNA
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)
Citrate - Isocitrate - alpha ketoglutarate - Succinyl CoA - Succinate - Fumarate - Malate - Oxaloacetate
Meconium ileus and failure to thrive; negative serum immunoreactive trypsin

40. What are the four fates of pyruvate metabolism?
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
An AP (apyrimidinic) endonuclease to cut the DNA - exonuclease then removes and then filled and sealed by enzymes
high Vmax
Insertion or deletion of a number of nucleotides indivisible by 3; tay sachs - duchennes MD

41. Where is anaerobic glycolysis used often?
Macrocytic megaloblastic anemia; it is the most common vit def in the US; no neuro symptoms and no methylmalonyl coA in the urine
RBCs - renal medulla - leukocytes - lens - testes - cornea
degradation
CCA and chemically modified bases Where amino acid is covalently bonded

42. What is unique about the diarrhea in lactase deficiency?
Bacteria produce lactate acid and gases from lactose and produce acidic diarrhea and bloating
Because carbamaoyl phosphate is involved in both urea cycle and pyrimidine synthesis
Aromatic; musty body odor
Phosphorylation of glucose

43. What are the causes of folate def?
Respiratory burst - P450 - and glutathione reductase
plasma membrane
Homocystinuria (homocysteine damages endothelial cells); alkaptonuria (tyrosine degradation); homocystinuria
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

44. How do you differentiate the causes of orotic aciduria?
Hydroxurea; deoxygenates ribose
Arylsulfatase A; cerebroside sulfate
Whether or not the patient is also hyperammonemia (if yes - then OTC def)
Riboflavin; dairy and meat

45. What carries electrons (as energy)?
Chorea and spastic paresis
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
Starts before age 5 with with pelvic girdle weakness with use of Gowers maneuver to stand up - it then progresses superiorly with muscle weakness (type 1 and 2!); waddling duck gait; hyporeflexia; pseudohypertrophy of calf muscles d/t fibrofatty repl
NADH - NADPH - FADH2

46. What lysosomal storage diseases are seen more in Ashkenazi Jew?
Tay Sachs - Niemann Picks - Gauchers
Decreased substrate for gluconeogenesis (pyruvate to lactate and not oxaloacetate); and pyruvate doing only lactic acidosis and not glycolysis
plasma membrane
III

47. ______________ converts tyrosine to DOPA
Exons
tyrosine hydroxylase
Mucus secreting goblet cells - ab secreting plasma cells - nt secreting neurons
Phenylalanine build up

48. in What disease are there antibodies to snRNPs?
Night blindness - dry skin
Lupus
TPP (from B1- pyrophosphate) - FAD (B2) - NADH2 (B3) - CoA (B5 - panothenate) - Lipoic Acid
be anorexic

49. What is the difference between wet and dry beriberi?
AD
The products of FA oxidation (Acetyl CoA and NADH) phosphorylate pyruvate deyhdrogenase Which inactivates it
30S + 50S= 70S
Dry is just the symmetrical neuropathy with both motor and sensory symptoms; wet includes both the neuropathy and cardiac involvement of high output failure with peripheral edema - tachycardia - cardiomegaly

50. What are two examples of X linked Autosomal Dominant disease?
Isocitrate dehydrogenase
Ascites from liver damage
Hypophosphatemia rickets and Alports
hexokinase - glucokinase (its a glutton)