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

2. From What aa do the catecholamines come from? and What aa does that aa come from?
Tyrosine; phenylaline
All clathrin
Vit C
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

3. What is the treatment for cystinuria?
Genotyping for mutant alleles (different sizes)
Acetazolamide to alkalinize the urine
Acidic
32 or 30; malate aspartate shuttle or glycerol 3 phosphate shuttle; heart/liver and muscle

4. What are causes of zinc deficiencies?
Alcoholism - RA - inflammatory disease - chronic diarrhea
Phenylethanolamine N- methyl transferase; SAM
AD
2 NADH - 2 ATP - 2 pyruvate - 2H+ - 2H20

5. High blood levels of fructose - galactose - glucose can result in conversion to osmotically active alcohol forms by...
Riboflavin makes up FAD and FMN both used in redox reactions; vegans
Blood group types
aldose reductase
Thick mucus plugs lungs recurrent pulmonary infections (pseudomonas and aureus) - chronic bronchitis - bronchiectasis (which can lead to cor pulmonale!); biofilms form on resp tract

6. What does the excess NH4+ in hyperammonia eat up? What is the result?
Cafe au lait spots - optic pathway gliomas - lisch nodules - scoliosis - neural tumors
Fatty acid (acyl coA) oxidation to acetyl coA; increase in dicarboxylic acids - and decreased glucose and ketones
Alpha ketoglutarate; no TCA cycle intermediates
Riboflavin makes up FAD and FMN both used in redox reactions; vegans

7. GMP is degraded to guanosine Which is degraded to guanine. What two options are next? by What enzymes?
Either salvage by HGPRT and PRPP or converstion to xanthine and then to uric acid by xanthine oxidase
Severe fasting hypoglycemia - very high levels glycogen in liver - high blood lactate - hepatomegaly
cell cycle events
In the reproductive tract and in the resp tract; infertility - bronchiectasis and recurrent sinusitis - associated with situs inversus

8. AR disease are usually...
Chronic pancreatitis leads to type 1 diabetes - pancreatic insufficiency leads to ADEK deficiencies - malabsorption and steatorrhea - blocked gallbladder ducts leads to secondary biliary cirrhosis
2/3; LCAT (lecithin cholesterol acyltransferase)
enzymatic
Carnitine acyltransferase I

9. What does fructose 6 phosphate do - depending On what?
Degeneration of retinal ganglion cells and axons; acute loss of central vision; mit. inheritance
This depends on if phosphofructokinase 1 is active and this depends on whether or not fructose 2.6 bisphophate is activating it or not - this can only occur if PFK 2 is on and not FBPase 2. then fructose 6 phosphate will participate in glycolysis. Ot
Assist in spontaneous refolding of proteins
Pellagra

10. What is uniparental disomy? give an example of a disease it can cause.
Vitamin B1 (thiamine) - Vitamin B2 (riboflacin) - Vitamin B3 (niacin) - Vitamin B5 (pantothenate) - Vitamin B6 (pyridoxine) - Vitamin B7 (Biotin) - Vitamin B12 (cobalamin) - folate - Vit. C
Codons may encode same amino acid since there are only ~20 aa
Biotin; rate limiting step in fA synthesis - acetyl coA into malonyl coA
When child gets two chromosomes from one parent; imprinting dx

11. What is the difference between exonucleases and endonucleases?
Endonucleases are specific and break specific linking phosphodiester bonds - exonucleases are not sequence specific and just remove the nucleoside
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)
1) Dry beriberi: peripheral neuropathy demyelination (symmetricul muscle wasting) 2) Wernicke: ataxia - confusion - nystagmus - mammilary body and medial dorsal nuc of thalamus hemorrhage 3) Korsakoff: anterograde and retrograde amnesia - demyelinati
Glycine; proline and lysine

12. Which vesicular trafficking protein takes vesicles from the Golgi to the ER (retrograde)?
Osteomalacia is vit D def seen in adults and causes bowed legs and results in increased fractures
COP I
2 NADH - 2 ATP - 2 pyruvate - 2H+ - 2H20
Phenylalanine build up

13. Other than to glycolysis - Where does glucose 6 phosphate get used for? ie What other activated carriers transmit its energy?
Citrate - Isocitrate - alpha ketoglutarate - Succinyl CoA - Succinate - Fumarate - Malate - Oxaloacetate
'9+2' 9 pairs of microtubules connected by an axonemal dynein ATPase in a cylinder with 20 MTs in the center; the connecting ATPase allows bending and differential sliding
Tyrosine
HMP shunt (to NADPH) and glycogenolysis (as glucose 1 phosphate)

14. What carries CO2 as energy?
Bilateral acoustic schwannomas - juvenile cataracts
Antineoplastic agents; alkylate DNA!; cyclophosphamide and nitrosurea (aka lomustrine - a nitrosource)
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
Biotin

15. What is the treatment of hyperammonia?
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)
Cells that stay in G0 and cannot replicate - instead they regenerate from stem cells; neurons - skeletal and cardiac muscles - and RBCs
Coenzyme A and lipoamides
Presence of normal and mutated mtDNA (like mosaicism except in mit)

16. What is the precursor for both purines?
Hungtington
Hepatic fatty change
2 oxidative and nonoxidative; cytoplasm (both); none (none produced either - you are simply switching carriers of energy- from glucose 6 phosphate to NADPH)
IMP

17. Where are ketone bodies made? What is the rate limiting enzyme? What are they made from?
Aminoglycosides and tetracyclines
Microvilli - muscle contraction - cytokinesis - adherens junctions
Mitochondria in liver; HmG CoA synthetase; amino acids and beta oxidation of FA
Cardiomegaly and systemic findings leading to early death; lysosomal alpha -1 -4- glucosidase (acid maltase)

18. Odd chain fatty acids yield one proprionyl coA which can enter the TCA cycle as...
succinyl coA (and then to OXA)
Carnitine shuttle
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)
Decreased serum phosphate

19. What is osmotic damage and What molecules cause it? give some examples of osmotic damage - in What disease state do you see these?
Griseofulvin
When water is pulled in; polyols (sugar alcohols); cataracts - retinopathy - peripheral neuropathy all seen in chronic hyperglycemia in diabetes
Phenylalanine build up
Methionine is only encoded by AUG

20. What is required to get Acyl coA in FA degradation into the mitochondria for beta oxidation to acetyl coA groups?
plasma membrane
Carnitine shuttle
Neurons and RBCs
3; 5; mit matrix; alpha ketoglutarate dehydrogenase complex

21. What aa becomes essential with homocystinuria? phenylalaninuria?
Cu; hydroxylation
A cytoskeletal protein - apart of flagella - cilia - mitotic spindles and transport (in neurons for example)
This depends on if phosphofructokinase 1 is active and this depends on whether or not fructose 2.6 bisphophate is activating it or not - this can only occur if PFK 2 is on and not FBPase 2. then fructose 6 phosphate will participate in glycolysis. Ot
Cysteine; tyrosine

22. Kwashiokor patients are not...
normal lactate levels
glucagon - insulin and epinephrine
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
calorie deficient

23. AD disease...
usually present late
Citrate - Isocitrate - alpha ketoglutarate - Succinyl CoA - Succinate - Fumarate - Malate - Oxaloacetate
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)
Metabolizes them with transferase enzymes to acetyl coA

24. After citrulline is produced and in the cytoplasm What happens next in the urea cycle?
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
Neural tissue and adrenal medulla
Endonucleases - exonucleases - DNA polymerase - DNA ligase
Avidin in raw egg whites prevents their reabsorption; also antibiotic use; dermatitis - alopecia - glossitis - lactic acidosis - enteritis

25. What structures allows for degeneracy?
TRNA wobble
Release alk phos
Chloramphenicol
Imprinting Dx's: prader Willi and Angelmann

26. What is the swollen belly from in a child with kwashiokor?
Cheilosis and corneal vascularization
Codons may encode same amino acid since there are only ~20 aa
Ascites from liver damage
CNS defects - mental retardation - death - feeding difficulties - vomiting - hypoglycemia; thiamine

27. In What cells does the Respiratory burst occur? Where is NADPH located in the cell? What is this reaction important for?
Pompes
Acetazolamide to alkalinize the urine
Neutrophils and monocytes; membrane bound; immune response by generating Reactive oxygen species
Multiple bone fractures - blue sclerae - hearing loss (abdnormal middle ear bones!) - dental imperfections - retarded wound healing

28. What is the end of anaerobic glycolysis?
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)
activation of PFK1
Pyruvate! (not lactate - that is generate just in order to produce more NAD+)
RER in nucleus (synthesize NTs)

29. What step in pyrimidine synthesis requires aspartate?
Peripheral neuropathies of hands/feet; angiokeratomas between umbilicus and knees; cardiovascular/renal disease
Carbamoyl phosphate to orotic acid
Abetalipoproteinemia and increased G3P production from all the carbs!
Self mutilation - aggression - mental retardation - gout - hyperuricemia - choreathetosis

30. What does glycogen phosphorylase in glycogenolysis create?
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
No
Methionine is only encoded by AUG
Limit dextrans

31. What are the results of the pregnancy screen for Downs?
Phenylaline
Decrease AFP - increased BCHG - decreased estriol - increased inhibin A
DHF
Both AD

32. What is the most common human enzyme deficiency? why? What is the mode of inheritance of this deficiency?
Dactinomycin - doxorubicin
Adipose tissue for brain and heart (ketone bodies) and then go to vital protein degradation (organ failure)
Neural tissue and adrenal medulla
G6PD; provides malarial resistance; X linked recessive

33. From Where is alk phos release in the bone? What enzyme does it act on? What is the result?
Glycogen phosphorylase
Phenylalanine build up
Neimann pick and tay sachs
Osteoblasts; de phosphorylates pyrophosphate Which inhibits bone mineralization; bone mineralization

34. What is a robertsonian translocation? What does it mean if its balanced?
They can enter G1 from G0 and replicate if they like; hepatocytes - lymphocytes - fibroblasts
Its When two acrocentric chromosomes combine their long arms (and short arms disappear); When the translocation is functional
Lens and neural tissue; converts galactose to osmotically active galactitol
Proprionyl coA; through methylmalonyl coA; B12

35. How many enzymes does the pyruvate dehydrogenase complex have? and How many cofactors? Where is it located? to What enzyme is it similar to?
Succinate hydrogenase; matrix
Alcohol dehyrogenase (alcohol to acetaldehyde); acetaldehyde dehydrogenase (acetaldehyde to acetate); NAD+; B3 (niacin)
3; 5; mit matrix; alpha ketoglutarate dehydrogenase complex
golgi apparatus

36. What is the effect of the Cardiac glycosides?
BM; laminins; most flexible; different aa composition
ATP synthase - ATP- ADP translocase - ETC
Thymidylate synthase; 5- Flourouracil
Indirect inhibtion of the Na+/Ca+2 exchange resulting in increased intracellular calcium Which increases cardiac contractility

37. What inheritance pattern is Type IIA hypercholesterolemia? do heterozygotes present with disease? do homozygotes present?
Adrenal medulla
Self mutilation - aggression - mental retardation - gout - hyperuricemia - choreathetosis
AD: yes! most common presentation; very rare (700 mg/dl cholesterol level!)
Some viruses

38. What is the function of B2? What can cause deficiency?
A 16C FA! the product in FA synthesis; saturated; no double bonds
Riboflavin makes up FAD and FMN both used in redox reactions; vegans
Type IV collagen defect; X linked recessive; hereditary glomerulonephritis - deafness - ocular disturbances
Via fructose 1 P by fructokinase in the liver; as glucose 1 by being GALT- ed ;)

39. 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
Citrate shuttle in the inner mitochondrial membrane transports acetyl coA into the cytoplasm; liver; liver and adipose tissue
Isocitrate dehydrogenase
When the disease manifestation depends on from who the gene deletion was inherited from d/t methylation that occurs; Prader Willi and AngelMann

40. Citrate shuttle; carnitine shuttle takes acyl coA into the mit for...
Myotonic Dystrophy (CTG) - Huntington (CAG) Friedricks Ataxia (GAA) - Fragile (CGG)
Glycogen and FFA oxidation - glucose conserved for final sprinting!
Decreased would healing; microcytic anemia (d/t decreased iron [not absorbable]; macrocytic anemia (d/t Fh4 not reduced)
fatty acid degradation

41. What disease is associated with chromosome 3?
VHL (Von Hippa Lindau)
Methionine; f Met (formyl methyl methionine)
Edwards
O2 consumption (last electron acceptor) gets consumed also

42. How many RNA polymerases do prokaryotes have? What drug blocks prokaryotic RNA polymerase only? What is it used for?
Defective neutral amino acid transporter on renal and intestinal cells; AR
Electron transport inhibitors - ATPase inhibitors - Uncoupling agents (increase permeability of the membrane - loss of hydrogen gradient)
1 for all three classes of RNA; Rifampin; TB
Cystine kidney stones (cystine staghorn calculi)

43. What is most often mutated in xeroderma pigmentosum? What is the inheritance pattern? What are the clinical symptoms?
Cell signaling defect in FGF receptor resulting in dwarfism and short limbs with normal head and trunk; AD
UV specific endonucleases for excision repair; AR; dry skin - photosensitivity - poikiloderma - susceptibility to skin cancers
Synthesis in liver and then hydrolysis of chylomicrons and VLDLs by capillary lipoprotein lipase in peripheral tissue; adipose tissue by lipases
Dihydropterin reductase; NADP+; phenylalanine to tyrosine and tyrosine to DOP

44. What is a positive allosteric regulator on pyruvate carboxylase? for What reaction is this?
Genotyping for mutant alleles (different sizes)
Acetyl coA; gluconeogenesis
Thick mucus plugs lungs recurrent pulmonary infections (pseudomonas and aureus) - chronic bronchitis - bronchiectasis (which can lead to cor pulmonale!); biofilms form on resp tract
Arylsulfatase A; cerebroside sulfate

45. What four enzymes are required for nucleotide excision repair?
Endonucleases - exonucleases - DNA polymerase - DNA ligase
Fructokinase; benign - fructose in the blood and urine
AD
Degeneration of retinal ganglion cells and axons; acute loss of central vision; mit. inheritance

46. What does lipolysis generate? and What two regulators activate lipases?
In hepatocytes; B6
Progressive neurodegeneration (spasticity d/t UMN disease) - NO hepatosplenomegaly - cherry red spot on macula - lysosomes with onion skin - muscle weakness
Triacylglycerols in adipose tissue broken down into glycerol and free fatty acids; growth hormone and epinephrine
Ehler Danlos; type III collagen def; joint dislocation - berry aneurysm - ecchymoses - organ rupture

47. What happens outside the fibroblast to procollagen?
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
Cytoplasmic side; active
AD: yes! most common presentation; very rare (700 mg/dl cholesterol level!)
low Vmax

48. What are the symptoms of arsenic poisoning?
Presence of normal and mutated mtDNA (like mosaicism except in mit)
Vomiting - rice water stools - garlic breath
NADH - Acetyl CoA - ATP; NAD+ - ADP and Ca2+
Homogentistic acid oxidase

49. What is the importance of folic acid?
COMT and MAO
Phenylketones that build up in phenylketonuria
Prevents neural tube defects in 1st trimester
Down's Syndrome

50. Which catecholamine has inhibitory action against acetylcholine?
Down's Syndrome
In ability to reabsorb phosphate from renal tubular cells results in osteomalacia due to defective bone mineralization; no
Dopamine
Ehler Danlos; type III collagen def; joint dislocation - berry aneurysm - ecchymoses - organ rupture