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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. Achondroplasia is associated with advanced ________ age
Milder form of type 1 with normal blood lactate levels; debranching enzyme (alpha -1 -6 glucosidase)
Hydroxurea; deoxygenates ribose
paternal
Inbit Na+K+ ATPase; Cardiac glycosides

2. What is uniparental disomy? give an example of a disease it can cause.
usually present late
Block ATP synthase resulting in no ATP production and an increased proton gradient
When child gets two chromosomes from one parent; imprinting dx
Alcohol dehyrogenase (alcohol to acetaldehyde); acetaldehyde dehydrogenase (acetaldehyde to acetate); NAD+; B3 (niacin)

3. What is vitamin B7? What is its main job? in What reactions?
(cofactor for 100+ enzymes!) needed for zinc fingers transcription motif - collagen synthesis - alcohol metabolism
Because carbamaoyl phosphate is involved in both urea cycle and pyrimidine synthesis
Biotin; cofactor for carboxylation; pyruvate carboxylase (turning pyruvate to oxaloacetate) - acetyl coA carboxylase (acetyl coA to malonyl Co A) - and proprionyl coA caroboxylase for (propionyl CoA to methylmalonyl coA)
Mitochondria - archaebacteria - mycoplasma - and some yeasts

4. What vit def occur from antibiotic use?
Biotin (vit B7) and Vit K
Microvilli - muscle contraction - cytokinesis - adherens junctions
Methionine is only encoded by AUG
Either salvage by HGPRT and PRPP or conversion to xanthine and then uric acid by xanthine oxidase

5. In the genetic code - What does unambiguous mean?
Block Electron transport
Each codon specifies only one amino acid
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
Beckers is less severe (instead of deletion is either defect or deficiency) with later presentation

6. What does a def in folic acid cause? is it more or less common than cobalamin def? How do you differentiate it with cobalamin def?
Chronic pancreatitis leads to type 1 diabetes - pancreatic insufficiency leads to ADEK deficiencies - malabsorption and steatorrhea - blocked gallbladder ducts leads to secondary biliary cirrhosis
Glutamate
Macrocytic megaloblastic anemia; it is the most common vit def in the US; no neuro symptoms and no methylmalonyl coA in the urine
Succinyl coA

7. What luminal secretions are affected in CF? What is wrong with the secretions?
To increase serum Ca and phosphate for bone deposition
Respiratory - seminal fluid - pancreatic - gallbladder; very thick without NaCl to pull water
DHF
So that excess glucose can be stored (phosphorylated glucose cant leave) in the liver and saved as a buffer

8. Increased levels of cAMP results in...
fructose
adrenal medulla
Arg and his
glycogenolysis

9. Loss of heterozygosity is the rule for...
Either salvage by HGPRT and PRPP or conversion to xanthine and then uric acid by xanthine oxidase
Avidin in raw egg whites prevents their reabsorption; also antibiotic use; dermatitis - alopecia - glossitis - lactic acidosis - enteritis
Respiratory burst - P450 - and glutathione reductase
tumor suppressor genes (p53 - Rb - BAX)

10. Where does splicing occur?
Excreted from the kidney in the form of urea in hepatocytes
Oxaloacetate depleted for gluconeogenesis; excess NADH shunts oxaloactetate towards malate
Vit C deficiency; glossitis - anemia - weakened vessels - hemorrahges - hemarthroses - swollen gums - bruises -
Nucleus

11. What does a pyridoxine deficiency look like? What causes it?
only yield acetyl coA
IMP
Convulsions - hyperirritiability - peripheral neuropathy - sideroblastic anemias; alcoholics and INH and fortified goats milk consumption
3 NADH - 1 FADH2 - 2 CO2 - 1 GTP = 12 ATP/acetyl CoA

12. _____________ prevents fluidity of plasma membrane
Do an RT PCR (reverse transcriptase)- RT the RNA first - then PCR the DNA
Microcephaly - mental retardation - high pitched mewing/crying - epicanthal folds - cardiac abnormalities; microdeletion on short arm of chrom 5
NADH - Acetyl CoA - ATP; NAD+ - ADP and Ca2+
high cholesterol

13. What cofactor is required of preprocollagen hydroxylation?
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
Lactic acidosis and ketoacidosis
VHL (Von Hippa Lindau)
Vit C

14. What enzyme is deficient in Fructose intolerance? What is the result of the enzyme def? What are the symptoms?
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
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
Aldolase B; Fructose 1 phosphate accumulates Which uses up Phosphate and the result is inhibition of gluconeogenesis without phosphate; hypoglycemia - jaundice - cirrhosis - vomitting
Synthesis in liver and then hydrolysis of chylomicrons and VLDLs by capillary lipoprotein lipase in peripheral tissue; adipose tissue by lipases

15. What carries CO2 as energy?
Adrenal cells (steroid synthesis) - hepatocytes (p450 detox)
Biotin
Lysine and leucine
Acetyl CoA carboxylase

16. What negatively regulates dopamine to norepinephrine?
Cafe au lait spots - optic pathway gliomas - lisch nodules - scoliosis - neural tumors
The products of FA oxidation (Acetyl CoA and NADH) phosphorylate pyruvate deyhdrogenase Which inactivates it
Carbidopa
Cri du Chat

17. After addition of an aa - Where does the aminoacyl tRNA now bind to?
Dermatitis - glossitis - diarrhea
30S ribosomal RNA (small one)
2/3; LCAT (lecithin cholesterol acyltransferase)
When the amino acid is covalently binded to its corresponding tRNA; Aminoacyl tRNA synthetase; ATP; translation

18. Do RNA polymerases have proofreading function? What does this mean for HIV?
No; increased mutation rate!
Creates nicks in DNA to prevent supercoiling; both!
Branching increases the rate of synthesis and breakdown of glycogen by having more sites to add on to and break off
Downs

19. What is the degradation product of Norepinephrine?
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
Fructose gets phosphorylated into fructose 1- P by fructokinase and then fructose 1P gets metabolized into DHAP or glyceraldehyde by aldolase B
Codons may encode same amino acid since there are only ~20 aa
VMA

20. What are labile cells? Which cells are labile?
Complex I - Complex II (lower energy)
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
Alpha amanitin; mushroom caps
Cells that never got into G0 and divide rapidly with a short G1; bone marrow - gut epithelium - hair follicles - skin

21. What is lebers hereditary optic neuropathy? What is the main clinical symptom? and What is its mode of inheritance?
Gauchers; hepatosplenomegaly - aseptic necrosis of the femur - bone crises - Gauchers cells - crumpled tissue paper looking (fibrillar appearing macrophages in liver - spleen - b.m.)
Wrinkles - acne and psoriasis topically - promyelocytic anemia - measles
Glucagon - insulin and epinephrine
Degeneration of retinal ganglion cells and axons; acute loss of central vision; mit. inheritance

22. Which oxidant is most attributable to damage of RBCs?
Proprionyl coA; through methylmalonyl coA; B12
LDL; LDL oxidize atherosclerosis
XR - iduronate sulfatase
Flouroquinolones

23. Findings for Cori's disease? deficient enzyme?
linked recessive
Carnitine shuttle
Respiratory - seminal fluid - pancreatic - gallbladder; very thick without NaCl to pull water
Milder form of type 1 with normal blood lactate levels; debranching enzyme (alpha -1 -6 glucosidase)

24. What is the purpose of the PCR?
Orotic acid in blood and urine (from pyrimidine synthesis) - decreased BUN - and symptoms of hyperammonemia
B12 and folate; B12; washed out
To amplify a specific desired fragment of DNA
Phenylketones that build up in phenylketonuria

25. How does glucagon activate glycogenolysis? What regulator does similar?
By activating adenylyl cylase Which increases cAMP Which increases levels of protein kinase A Which activates glycogen phosphorylase kinase; epinephrine
Mcardles disease
Microvilli - muscle contraction - cytokinesis - adherens junctions
Limit dextrans

26. What is type II collagen found?
Cartilage (including hyaline) - vitreous body - nucleus pulposus
Glutamine PRPP amidotransferase
Carbomyl phosphate synthetase II
golgi apparatus

27. Which are the mitochondrial reactions of the urea cycle?
Phenylaline
Carbomyl phosphate synthetase I turns N acetyl glutamate into carbomyl phosphate and then ornithine transcarbomyalse turns carbomyl phosphate into citrulline (using ornithine)
No
Fructose bisphosphatase 2

28. 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
Lime and CaCO3 to displace the niacin
alk phos
Fabrys Disease and Hunters Syndrome

29. After glucose 6 P is converted to glucose 1 P - What is the next step in glycogenesis? then what?
Presence of normal and mutated mtDNA (like mosaicism except in mit)
Conversion to UDP glucose via UDP glucose pyrophosphorylase; glycogen synthase (rate limiting step )
Biotin (vit B7) and Vit K
aldose reductase

30. The three steps of PCR
All and Alzheimers
Formation of a proton gradient that is then used to create ATP through ATP synthase
Pyruvate to lactate (results in lactic acidosis and decreased pyruvate for oxaloacetate for gluconeogenesis) - increased production of glycerol 3 phosphate from DHAP for TG synthesis; and increased betahydroxybutyrate ketoacid production from increas
1. denature by heating 2. during cooling anneal with DNA primers 3. elongation - add heat stable DNA polymerase 4. repeat

31. What is the rate determining enzyme in glycolysis?
Ret
Back up of pyruvate and alanine and thus lactic acidosis occurs; can be d/t alcoholism (vit B1 def)
Zero order; increased production of NADH no matter the amount of end products produced
Phosphofructokinase 1

32. in What organ is D3 converted from 25 OH D3? with What enzymes?
When the 2nd hit of the allele is What causes the disease; sporadic retinoblastoma
Bacterial synthesis in the intestine
Fructose bisphosphatase 2
Liver; P450 hydroxylation

33. What does fructose 6 phosphate do - depending On what?
Tyrosinase
Acetazolamide to alkalinize the urine
A topoisomerase II inhibitor; because cancer cells use II more so than healthy cells
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

34. Other than to glycolysis - Where does glucose 6 phosphate get used for? ie What other activated carriers transmit its energy?
A ketone
Inosine/IMP; adenosine deaminase
Gets converted to glycerol 3 P Which is then converted to DHAP Which then can enter glycolyis
HMP shunt (to NADPH) and glycogenolysis (as glucose 1 phosphate)

35. Where does the HMP (pentose phosphate pathway) occur other than in the liver?
Galactose 1 phosphate uridyltransferase (GALT) results in an accumulation of toxic substances; failure to thrive - jaundice - hepatomegaly - infantile cataracts - mental retardation; exclude galactose and lactose from diet
Synthesis in liver and then hydrolysis of chylomicrons and VLDLs by capillary lipoprotein lipase in peripheral tissue; adipose tissue by lipases
Lactating mammary glands - adrenal cortex (sites of fatty acid or steroid synthesis) - RBCS (for glutathione reduction for FR injury)
be anorexic

36. What is Maternal PKU?

37. Insulin acts on __________ to deactivate glycogenolysis
Ile - Phe - Thr - Trp
both liver and muscle (unlike glucagon)
To amplify a specific desired fragment of DNA
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

38. What aa is the ultimate precursor of catecholamine synthesis?
Phenylalanine
Gets converted to pyruvate via the Cori cycle in the liver
Mental and growth retardation; seizures - fair skin - eczema - musty body odor; neurotoxic ketones and acids stimulate vomitting - can lead to pyloric stenosis
IMP

39. in What disease are there antibodies to snRNPs?
Also have fair skin - blond hair - blue eyes - eczema
Liver mit and liver cytosol
Lupus
Fructose bisphosphatase 2

40. What do hexokinase and glucokinase do? using what? What pathway is this in?
Phosphofructokinase 1
the RER
A cytoskeletal protein - apart of flagella - cilia - mitotic spindles and transport (in neurons for example)
Phosphorylation of glucose to yield glucose 6 phosphate; ATP; first step of glycolysis

41. What are the functions of Microtubule proteins?
30S ribosomal RNA (small one)
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)
A cytoskeletal protein - apart of flagella - cilia - mitotic spindles and transport (in neurons for example)
Kidney and intestinal epithelium

42. What disease is associated with chromosome 5?
30S ribosomal RNA (small one)
Because that is Where the products will be consumed; acetyl coA; TCA cycle of ketone body production
Smoking (FR takes up all the antioxidants); diets lacking fruit
Familial Adenomatous Polyposis (APC gene)

43. polymerase chain reaction (PCR)- Name an application for it
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
glycogenolysis
The products of FA oxidation (Acetyl CoA and NADH) phosphorylate pyruvate deyhdrogenase Which inactivates it
Genotyping for mutant alleles (different sizes)

44. Because electron transport continues...
Alpha ketoacids in the blood; alpha keto acids in the urine/poo smells of maple syrup/burnt sugar
Gets converted to glycerol 3 P Which is then converted to DHAP Which then can enter glycolyis
1) glycogenolysis (liver only contributes to plasma - muscle keeps it for itself) 2) gluconeogenesis
O2 consumption (last electron acceptor) gets consumed also

45. Glucokinase is a...
glutton
Fructose bisphosphatase 2 and Phosphofructokinase 2; phosphorylation by protein kinase A; inuslin/glucagon ratio
can be d/t lyonization
Because carbamaoyl phosphate is involved in both urea cycle and pyrimidine synthesis

46. From What aa do NAD+ and NADP+ come from?
Double stranded RNA is created that is complementary to the mRNA of interest and When inserted into cells the strands separate and attach to the mRNA preventing it from being translated
2 NADH - 2 ATP - 2 pyruvate - 2H+ - 2H20
Tryptophan
Very Poor Carb Metabolism (Von Gierke - Type 1 - Pompes - Type II - Cori's - Type III - McArdles disease - Type IV)

47. What does deficiency of Vit K cause?
Limit dextrans
CPSII
Griseofulvin
Hemorrhage with increased PT and PTT time but normal bleeding time

48. What carries methyl groups as energy?
anabolic processes
Myotonic Dystrophy (CTG) - Huntington (CAG) Friedricks Ataxia (GAA) - Fragile (CGG)
Glucokinase
SAM

49. How is CF diagnosed?
'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
Niemann pick and Tay Sachs; hepatosplenomegaly or not
Increased concentration of Cl ions in sweat test
1) if it is stable/labile or permanent 2) intact BM and 3) intact ECM

50. What aa becomes essential with homocystinuria? phenylalaninuria?
Cysteine; tyrosine
Pantothenate; essential comp of CoA (needed for acyl transfers) and fatty acid synthase
Actin/myosin - MT - intermediate filaments
Von gierkes