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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 is the main form of glycogenolysis after glycogen phosphorylase? What happens instead in some lysosomes? does this process use glycogen phosphorylase? in What disease is this enzyme deficient?
Debranching enzyme; alpha 1 - 4 glucosidase; no!; Pompes disease
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)
Excreted from the kidney in the form of urea in hepatocytes
Fructose bisphosphatase 2

2. What causes hypoglycemia in alcoholism?
Decreased substrate for gluconeogenesis (pyruvate to lactate and not oxaloacetate); and pyruvate doing only lactic acidosis and not glycolysis
Fatty acid (acyl coA) oxidation to acetyl coA; increase in dicarboxylic acids - and decreased glucose and ketones
All clathrin
Endonucleases are specific and break specific linking phosphodiester bonds - exonucleases are not sequence specific and just remove the nucleoside

3. What is def of Vit E caused by?
Nacetyl cysteine; breaks disulfide bridges of mucus plugs
Fat malabsorption (CF) and abetalipoproteinemia (damage takes up all antioxidants)
Cystathionine (which then goes to cysteine) via cystathionine reductase; B6
Tyrosine

4. How does an enzyme know to go to lysosome?
Downs
5' capping (with 7 methylguanosine) - 3' adenylation - splicing of introns; nucleus
SER and mitochondria; NADPH
By addition of mannose 6 phosphate to proteins in the Golgi

5. What disease is associated with chromosome 5?
Metaphase (all lined up); blood - bone marrow - amniotic fluid - placental tissue; used to diagnose chromosomal abnormalities (deletions - trisomies - sex chromosomal disorders)
Cri du Chat
Decreased serum phosphate
A topoisomerase II inhibitor; because cancer cells use II more so than healthy cells

6. What Abs block the binding between aminoacyl tRNA and rRNA by binding the 30S ribosome?
linked recessive
Def in Vit C; def in Vit D
Aminoglycosides and tetracyclines
usually present late

7. What is the inheritance of myoclonic epilepsy?
Mitochondrial inheritance
Hydroxurea; deoxygenates ribose
Myotonic Dystrophy (CTG) - Huntington (CAG) Friedricks Ataxia (GAA) - Fragile (CGG)
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)

8. How many bonds does a G- C connection have? What type of bonds? how about a A- T?
3 hydrogen bonds; 2 hydrogen bonds
Arginne
Milder form of type 1 with normal blood lactate levels; debranching enzyme (alpha -1 -6 glucosidase)
jaundice

9. What is there on increased risk of with Downs?
show disease
Indirect inhibtion of the Na+/Ca+2 exchange resulting in increased intracellular calcium Which increases cardiac contractility
Trinucleotide expansion diseases
All and Alzheimers

10. What two def. most commonly cause phenylketonuria?
APCKD
Adrenal medulla
Phenylalanine hydroxylase or tetrahydrobiopterin factor
P53 and Rb; p53 activates BAX Which inhibits BCL2 When there is DNA damage - Rb phosphorylation allows progression into S phase

11. What converts fructose 2 -6 phosphate back to fructose 2 phosphate?
liver
Down's Syndrome
Fructose bisphosphatase 2
Bilateral acoustic schwannomas - juvenile cataracts

12. CFTR channel ______ Cl into sweat - secretes NaCl in luminal secretions
Glycogen phosphorylase; phosphorylation and dephosphorylation
reabsorbs
Stored ATP - creatine phosphate - anaerobic glycolysis; as distances increase - ATP is obtained from additional resources; above plus ox phos
Carnitine shuttle

13. What aa becomes essential with phenylketonuria?
Vit C
Glutamine - aspartate - glycine; aspartate
Tyrosine
HVA

14. When is phenylketonuria screened? why?
2-3 days after birth because maternal enzyme during fetal like might still be there
glucagon - insulin and epinephrine
Kartaganer's aka ciliar dyskinesia; lack of dynein arm ATPase
Peripheral neuropathy - developmental delay - optic atrophy - globoid cells

15. What are heat shock proteins?
Assist in spontaneous refolding of proteins
Chromosomal imbalance (downs for ex)
Cylindrical made up of a helical array of polymerized alpha and beta tubulin dimers - each dimer has 2 GTP bound
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)

16. What is an ELISA? What are the two ways to perform it? What is the sensitive and specificity? give a relative clinical example.
Glycogen and FFA oxidation - glucose conserved for final sprinting!
Collagen; fibroblasts; 4; ECM
Testing for antigen - antibody reactivity; can either add an antibody to a persons blood sample to see if there is binding or add an antigen to a persons blood sample to see if there immune system recognizes it; close to 100% for both; HIV eliza - an
Sulfonamides - primaquine - dapsone - antiTB drugs - fava beans - LDL; RBCs

17. Other than anabolic processes What else is NADPH used for?
Night blindness - dry skin
Hemolytic anemia due to heinz bodies Which are oxidized hemoglobin precipitating within the RBCs
Respiratory burst - P450 - and glutathione reductase
Liver mit and liver cytosol

18. What is lebers hereditary optic neuropathy? What is the main clinical symptom? and What is its mode of inheritance?
SER and mitochondria; NADPH
Riboflavin makes up FAD and FMN both used in redox reactions; vegans
Both AD
Degeneration of retinal ganglion cells and axons; acute loss of central vision; mit. inheritance

19. What lysosomal storage diseases are seen more in Ashkenazi Jew?
Tay Sachs - Niemann Picks - Gauchers
Cystic Fibrosis (CFTR gene)
Proteolytic processing (of water soluble parts to less soluble) to turn into troprocollagen and then crosslinking between hydroxylated lysine residues by lysyl oxidase to make collagen fibrils
Liver and skeletal muscle

20. What is the degradation product of dopamine?
HVA
Familial Adenomatous Polyposis (APC gene)
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
enzymatic

21. What causes fruity odor in ketoacidosis?
Back up of pyruvate and alanine and thus lactic acidosis occurs; can be d/t alcoholism (vit B1 def)
Von gierkes
Indirect inhibtion of the Na+/Ca+2 exchange resulting in increased intracellular calcium Which increases cardiac contractility
Acetone

22. What carries electrons (as energy)?
Aldolase B; Fructose 1 phosphate accumulates Which uses up Phosphate and the result is inhibition of gluconeogenesis without phosphate; hypoglycemia - jaundice - cirrhosis - vomitting
NADH - NADPH - FADH2
Tyrosine hydroxylase; tetrahydrobiopterin factor; NADP+
Tyrosine; phenylaline

23. How does glycerol from triacylglycerols in adipose tissue used for gluconeogenesis?
Triacylglycerols in adipose tissue broken down into glycerol and free fatty acids; growth hormone and epinephrine
Blood group types
A topoisomerase II inhibitor; because cancer cells use II more so than healthy cells
Gets converted to glycerol 3 P Which is then converted to DHAP Which then can enter glycolyis

24. What is the inheritance pattern of hereditary spherocytosis?
Heterochromatin; euchromatin
To provide NADPH since there is an abundance of glucose 6 phosphate; also yields ribose for nucleotide synthesis and glycolysis intermediates
AD
Carbomyl phosphate synthetase I

25. What are the findings in Krabbes disease?
Autosomal Dominant; NF1 and mit. diseases (variable expression) - ?
Arginine
Peripheral neuropathy - developmental delay - optic atrophy - globoid cells
A topoisomerase II

26. What does deficiency of Vit K cause?
Increased blood levels of ammonia - decreased BUN
2/3; LCAT (lecithin cholesterol acyltransferase)
Hemorrhage with increased PT and PTT time but normal bleeding time
TCA cycle - FA oxidation - acetyl coA production; ox phos

27. What three things contribute to fatty change in the liver in alcoholism?
Heterochromatin; euchromatin
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
Bendy bones seen in children due to decreased Vit D; craniotabes - skeletal deformities - growth retardation - rosary
Uric acidemia; HGPRT; purine salvage

28. What disease is associated with chromosome 7?
NADH - NADPH - FADH2
Cystic Fibrosis (CFTR gene)
Catalyze gamma carboxylation of glutamic acid residues on coag factors so they can bind to Ca complex
Citrulline is combined with aspartate to make arginosuccinate Which is then split into fumarate (which leaves to enter TCA) and arginine. Arginine is then converted to Ornithine (via arginase) giving off Urea Which then goes to the kidney and ornithi

29. All the glycogen storage diseases...
Carnitine shuttle
By activating protein phosphatase Which both dephosphorylates glycogen phosphorylase (deactivating) and dephosphorylates glycogen synthase (activating it )
are AR
Back up of pyruvate and alanine and thus lactic acidosis occurs; can be d/t alcoholism (vit B1 def)

30. What is retinoic acid used for in treatments?
Severe fasting hypoglycemia - very high levels glycogen in liver - high blood lactate - hepatomegaly
Lipoprotein lipases take FA from VLDL and chylomicrons Which are then combined with glycerol 3 P (a carb intermediate) to make triacylglycerol
Wrinkles - acne and psoriasis topically - promyelocytic anemia - measles
Excess ATP and dATP has negative feedback inhibition on ribonucleotide reductase Which results in decreased DNA synthesis and thus decreased lymphocyte count

31. What causes scurvy? What are the symptoms of scurvy?
AR
COMT and MAO
S - adenosyl methionine
Vit C deficiency; glossitis - anemia - weakened vessels - hemorrahges - hemarthroses - swollen gums - bruises -

32. Where is there decreased hydrogen concentration in the mit? increased?
H1
Heterochromatin; euchromatin
XR - iduronate sulfatase
Matrix; intermembrane space

33. By What products are gluconeogenesis and glycolysis linked so that they are not both occurirng at the same time?
Glycogen synthase
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
By addition of mannose 6 phosphate to proteins in the Golgi
Ascites from liver damage

34. What are the clinical symptoms of Marfans?
Nucleoside is just the ribose plus the base; nucleotide is the ribose plus the base but the phosphate bond
Heart - SKELETON - and EYES: arachnodactyly - long limbs - cystic medial necrosis of aorta resulting in aneurysms (and eventual dissection) - floppy mitral valve (prolapse and regurg!) - subluxation of lens
Cell signaling defect in FGF receptor resulting in dwarfism and short limbs with normal head and trunk; AD
Phenylketones that build up in phenylketonuria

35. What kind of DNA damage does ionizing radiation cause? What type of repair helps with this?
All clathrin
Essential fructosuria and fructose intolerance; fructose intolerance
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
Double strand breaks; nonhomologous end joining

36. What is the rate determining enzyme in de novo purine synthesis?
AD
acetyl coA
So that excess glucose can be stored (phosphorylated glucose cant leave) in the liver and saved as a buffer
Glutamine PRPP amidotransferase

37. High blood levels of fructose - galactose - glucose can result in conversion to osmotically active alcohol forms by...
Genotyping for mutant alleles (different sizes)
Proprionyl coA; through methylmalonyl coA; B12
Induction in chronic alcoholism (increases tolerance); inhibition in acute alcoholism
aldose reductase

38. Findings for McArdle's disease? deficient enzyme?
5' capping (with 7 methylguanosine) - 3' adenylation - splicing of introns; nucleus
Prevents neural tube defects in 1st trimester
Phenylalanine hydroxylase; tetrahydrobiopterin factor
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

39. What drug blocks acetaldehyde dehyrogenase? What are its side effects?
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)
Branching increases the rate of synthesis and breakdown of glycogen by having more sites to add on to and break off
When different combination of exons get combined together; in Beta thallasemia
Disulfiram (antabuse); acetaldehyde accumulate contributing to hangover

40. Which end carries the triphosphate? Which end of DNA makes the hydoxyl attack?

41. Other than to glycolysis - Where does glucose 6 phosphate get used for? ie What other activated carriers transmit its energy?
HMP shunt (to NADPH) and glycogenolysis (as glucose 1 phosphate)
Either salvage by HGPRT and PRPP or conversion to xanthine and then uric acid by xanthine oxidase
Mental retardation - rocker bottom feet - micropthalmia - microcephaly - cleft lip/palate - holoprosoncephaly - polydactyly
Galactocerebrosidase; Galactocerebroside

42. 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
The lack of lysosomal proteins (are secreted - default from RER!) results in macromolecules accumulating in lysosomes - inclusions cause pathology
Marfans - homocystinuria - MEN 2B
Thinned cornea from lack of connective tissue so see underlying choroidal lens

43. What is carnitine deficiency and What does it result in?
An AP (apyrimidinic) endonuclease to cut the DNA - exonuclease then removes and then filled and sealed by enzymes
NADPH- reductive biosynthesis!
Inability to transport LCFA into mitochondria results in their build up and causes weakness - hypotonia And HYPOKETOTIC HYPOGLYCEMIA
Chloramphenicol

44. What is the metabolism of fructose?
Fructose gets phosphorylated into fructose 1- P by fructokinase and then fructose 1P gets metabolized into DHAP or glyceraldehyde by aldolase B
Increased acid production competes with uric acid for excretion
Pulmonary - GI - reproductive (infertility in male due to absence of vas deferens and seminal fluid thickness)
Conversion of pyruvate to Acetyl CoA; acetyl coA - CO2 - and NADH (one of each per pyruvate)

45. What is the mode of inheritance of the muscular dystrophies? What is the pathogenesis of the disease?
Microvilli - muscle contraction - cytokinesis - adherens junctions
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
Microcephaly - mental retardation - high pitched mewing/crying - epicanthal folds - cardiac abnormalities; microdeletion on short arm of chrom 5
30S ribosomal RNA (small one)

46. How would one use a PCR to id an RNA virus?
Do an RT PCR (reverse transcriptase)- RT the RNA first - then PCR the DNA
AD: yes! most common presentation; very rare (700 mg/dl cholesterol level!)
Glucosekinase to glucose 6 P
ATP and biotin

47. What does a deficiency cause?
Night blindness - dry skin
Tyrosinase deficiency; defective tyrosine transporters; lack of migration of neural crest cells; skin cancer
Thick mucus plugs lungs recurrent pulmonary infections (pseudomonas and aureus) - chronic bronchitis - bronchiectasis (which can lead to cor pulmonale!); biofilms form on resp tract
Kidney and intestinal epithelium

48. How does glucagon activate glycogenolysis? What regulator does similar?
Induction in chronic alcoholism (increases tolerance); inhibition in acute alcoholism
Thymidylate synthase; 5- Flourouracil
By activating adenylyl cylase Which increases cAMP Which increases levels of protein kinase A Which activates glycogen phosphorylase kinase; epinephrine
Methionine is only encoded by AUG

49. Which are the mitochondrial reactions of the urea cycle?
Carbomyl phosphate synthetase I turns N acetyl glutamate into carbomyl phosphate and then ornithine transcarbomyalse turns carbomyl phosphate into citrulline (using ornithine)
Citrulline is combined with aspartate to make arginosuccinate Which is then split into fumarate (which leaves to enter TCA) and arginine. Arginine is then converted to Ornithine (via arginase) giving off Urea Which then goes to the kidney and ornithi
Calcium and calmodulin activate glycogen phosphorylase kinase
Cells that never got into G0 and divide rapidly with a short G1; bone marrow - gut epithelium - hair follicles - skin

50. How does mismatch repair work? in What cancer is it mutated? What type of DNA is methylated?
Block ATP synthase resulting in no ATP production and an increased proton gradient
It is apart of thiamine pyrophosphate Which is a cofactor enzymes involved in ATP production: pyruvate dehydrogenase (glycolysis) - alpha ketoglutarate dehydrogenase (TCA cycle) - transketolase (HMP shunt) - branched chain AA dehydrogenase
Inability to transport LCFA into mitochondria results in their build up and causes weakness - hypotonia And HYPOKETOTIC HYPOGLYCEMIA
Immediately after replication - DNA is unmethylated and so it is a signal to be double checked; HNPCC; heterochromatin