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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. Which antigout drugs act on MT?
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
is active
Colchine
Citrate is positive on it - AMP and fructose 2 -6 bisphosphate is negative on it; gluconeogenesis

2. Insulin receptor is not cAMP second receptor but rater tyrosine kinase...
Which dimerizes and activates a protein phosphatase Which de phosphorylates glycogen phosphorylase
high Vmax
anabolic processes
Alpha ketoacids in the blood; alpha keto acids in the urine/poo smells of maple syrup/burnt sugar

3. AD diseases are usually...
pleiotropic
Phenylalanine hydroxylase; tetrahydrobiopterin factor
Griseofulvin
paternal

4. What drug blocks ribonucleotide reductase? What does this enzyme do?
Converts it to citrulline
Lysine and leucine
Neutrophils and monocytes; membrane bound; immune response by generating Reactive oxygen species
Hydroxurea; deoxygenates ribose

5. What causes an excess of B3? What are the symptoms?
Lysine and arginine; basic
Fructose 1 -6 bisphosphatase
So that excess glucose can be stored (phosphorylated glucose cant leave) in the liver and saved as a buffer
Niacin treatment for hyperlipidemia; facial flushing

6. other than succinate dehydrogenase What other enzymes are found in the inner mit membrane?
2 -4 dinitrophenol - aspirin - and thermogenin (in brown fat)
Uracil; thymine
Cell signaling defect in FGF receptor resulting in dwarfism and short limbs with normal head and trunk; AD
ATP synthase - ATP- ADP translocase - ETC

7. What are the clinical symptoms of duchennes muscular dystrophy?
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
only yield acetyl coA
Succinate hydrogenase; matrix
S. aureus - Aspergillus

8. What does the excess NH4+ in hyperammonia eat up? What is the result?
Alpha ketoglutarate; no TCA cycle intermediates
Inosine/IMP; adenosine deaminase
Water soluble vitamin stored in liver; converted to THF by dihydrofolate reductase and is a coenzyme in 1 methyl transfers - impt for DNA synth
Sphingomyelinase; sphingomyelin

9. What is the rate determining enzyme in de novo pyrimidine synthesis?
COP II
Carbomyl phosphate synthetase II
O2 consumption (last electron acceptor) gets consumed also
reabsorbs

10. Cysteine is one of two _____ connected by disulfide bond
glutton
liver
dimers
AR; trinucleotide deletion results in loss of a Phe aa on CFTR gene on chrom 7- the result is a mutated CFTR channel that gets degraded in the ER before reaching the surface; the purpose of CFTR is to increase Cl secretion in luminal secretions and r

11. What is the swollen belly from in a child with kwashiokor?
Alpha galactosidase A; ceramide trihexoside
long saturated fatty acids
Southern - DNA - Northern - RNA - Western - Protein; blotting procedures are used to detect a specific nucleic acid or aa sequence; the sample is electrophoresed and then transferred to a filter Which is then soaked in a denaturant and then a radiola
Ascites from liver damage

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

13. Breast milk has a little of...
Genotyping for mutant alleles (different sizes)
They can enter G1 from G0 and replicate if they like; hepatocytes - lymphocytes - fibroblasts
Vit D and K
HMG CoA reductase

14. Which anticancer drugs act on MT? how are they different?
Hurlers Syndromes;Iduronate sulfatase deficiency; alpha L iduronidase def; heparan sulfate - dermatan sulfate
Aldolase B; Fructose 1 phosphate accumulates Which uses up Phosphate and the result is inhibition of gluconeogenesis without phosphate; hypoglycemia - jaundice - cirrhosis - vomitting
Dilated capillaries and veins; skin - mucous membranes - GI - resp - urinary tracts; rupture and can cause epistaxis - hemorrhage - GI bleeding
Vincristine - vinblastine - and paclitaxel; the vinca alkaloids block formation of the spindle and paclitaxel prevents breakdown of the mT spindle

15. Vitamin c is needed in...
HnRNA; mRNA
Lens and neural tissue; converts galactose to osmotically active galactitol
Uric acidemia; HGPRT; purine salvage
catecholamine synthesis (dopamine to norepinephrine)

16. What is the storage form of vit D? active form?
Lactating mammary glands - adrenal cortex (sites of fatty acid or steroid synthesis) - RBCS (for glutathione reduction for FR injury)
25 OH D3; 1 -25 (OH)2 D3 (Calcitriol)
AD
Vitamin B1 (thiamine) - Vitamin B2 (riboflacin) - Vitamin B3 (niacin) - Vitamin B5 (pantothenate) - Vitamin B6 (pyridoxine) - Vitamin B7 (Biotin) - Vitamin B12 (cobalamin) - folate - Vit. C

17. What happens in an aminotransferase reaction
An aminotransferase enzyme transfers an amino group from an amino acid on to alpha ketoglutarate making it a glutamate (to join the urea cycle or excreted in the kidney) and the amino acid that lost the amino group is then turned into a glycolytic in
By activating protein phosphatase Which both dephosphorylates glycogen phosphorylase (deactivating) and dephosphorylates glycogen synthase (activating it )
Phosphofructokinase 1; fructose 1 phosphate to fructose 1 -6 bisphosphate; enhancers: AMP - fructose 2 -6 Bisphosphate; downregulators: citrate - ATP
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

18. Which antifungal acts on MT?
MT!
NADH - NADPH - FADH2
Griseofulvin
Dynein - retrograde - kinesin - anterograde

19. in Fabrys - What accumulates in tissue because fibroblasts express the def enzyme highly?
Ceramide trihexoside
Becomes methionine
Arginine
Abnormal glycogen metabolism results in accumulation of glycogen in cells

20. AD disease...
usually present late
Adrenal cells (steroid synthesis) - hepatocytes (p450 detox)
Reticulin - skin - blood vessels - uterus - fetal tissue - early phase of granulation tissue
Glucosekinase to glucose 6 P

21. What are the findings in maple syrup disease? What should be tried to be given as treatment?
Acetazolamide to alkalinize the urine
Glycosylases (only remove the base - cut the bond between ribose and base)
Pompes
CNS defects - mental retardation - death - feeding difficulties - vomiting - hypoglycemia; thiamine

22. From What aa does serotonin come from? What NT is serotonin a precursor for?
Arginne
Induction in chronic alcoholism (increases tolerance); inhibition in acute alcoholism
Tryptophan; melatonin
3'--> 5'; prokaryote only

23. What are the GI clinical symptoms of CF?
Progressive neurodegeneration (spasticity d/t UMN disease) - hepatosplenomegaly - cherry red spot on macula - foam cells
the RER
Chronic pancreatitis leads to type 1 diabetes - pancreatic insufficiency leads to ADEK deficiencies - malabsorption and steatorrhea - blocked gallbladder ducts leads to secondary biliary cirrhosis
Neurons and RBCs

24. What does cross linking to collagen do? What does this have to do with age?
Deficiencies in one of the many lysosomal enzymes results in accumulation of abnormal metabolic products
Helps make Vit K and antioxidant
3; 5; mit matrix; alpha ketoglutarate dehydrogenase complex
Make it stiffer/more tensile; continues as we get older so gets less and less elastic

25. What is the structure of Microtubules?
Later
Adrenal medulla
allosteric regulation
Cylindrical made up of a helical array of polymerized alpha and beta tubulin dimers - each dimer has 2 GTP bound

26. Other than glycogenesis and glycolysis - What else is Fructose used for?
By activating protein phosphatase Which both dephosphorylates glycogen phosphorylase (deactivating) and dephosphorylates glycogen synthase (activating it )
Mannose
Hypophosphatemia rickets and Alports
With glucose 6 phosphatase; Von Gierkes disease

27. In What genetic code is uracil found in? in replace of what?
Uracil; thymine
activator
Alcohol dehydrogenase; cytosol; mitochondria; NAD+
Niacin treatment for hyperlipidemia; facial flushing

28. What carries 1 carbon units as energy?
all are diseased!
Tetrahydrofolates
Phenylalanine hydroxylase or tetrahydrobiopterin factor
When different combination of exons get combined together; in Beta thallasemia

29. NAD+ is used in catabolic processes and NADPH is used in...
Because that is Where the products will be consumed; acetyl coA; TCA cycle of ketone body production
anabolic processes
Self mutilation - aggression - mental retardation - gout - hyperuricemia - choreathetosis
Lactose in mothers milk contains galactose

30. In the fasting state - What ensures that the pyruvate doesnt get used for TCA cycle?
The products of FA oxidation (Acetyl CoA and NADH) phosphorylate pyruvate deyhdrogenase Which inactivates it
DNA dependent RNA polymerase
Hemorrhage with increased PT and PTT time but normal bleeding time
Sulfonamides - primaquine - dapsone - antiTB drugs - fava beans - LDL; RBCs

31. Which antihelminthic drugs act on microtubules?
SAM
Phosphoenolpyruvate; pyruvate kinase; fructose 1 - 6 BP; ATP - alanine
can be d/t lyonization
Mebendazole and thiabendazole

32. What are the clinical symptoms of NF2?
N5N10 methylene THF
False; GTP hydrolysis
Osmotic diarrhea - bloating - cramps
Bilateral acoustic schwannomas - juvenile cataracts

33. What type of proteins have AD mode of inheritance?
Structural (constitutively active)
VMA
Vit C deficiency; glossitis - anemia - weakened vessels - hemorrahges - hemarthroses - swollen gums - bruises -
Vit C; Vit E

34. What is the precursor for pyrimidines?
NADH - Acetyl CoA - ATP; NAD+ - ADP and Ca2+
golgi apparatus
Orotate
AD: yes! most common presentation; very rare (700 mg/dl cholesterol level!)

35. What is most often mutated in xeroderma pigmentosum? What is the inheritance pattern? What are the clinical symptoms?
Galactocerebrosidase; Galactocerebroside
UV specific endonucleases for excision repair; AR; dry skin - photosensitivity - poikiloderma - susceptibility to skin cancers
Liver; P450 hydroxylation
adrenal medulla

36. How do you differentiate the causes of orotic aciduria?
homocysteine transferase
Whether or not the patient is also hyperammonemia (if yes - then OTC def)
All clathrin
Oxaloacetate depleted for gluconeogenesis; excess NADH shunts oxaloactetate towards malate

37. What is Maternal PKU?

38. What is the most abundant protein in the body? From What cell type is it made? How many types are there? Where is it found (generally)?
high Vmax
Collagen; fibroblasts; 4; ECM
Pyruvate dehydrogenase; ATP - NADH - acetyl CoA
With glucose 6 phosphatase; Von Gierkes disease

39. Homocysteine gets turned into methionine by...
homocysteine transferase
Loss of subcutaneous fat - tissue and muscle wasting (somatic muscle); visceral - not somatic
AD; disorder of blood vessels that results in telangietasias - skin discolorations - and AVMs (arteriovenous malformations)
Steroid synthesis and detoxification

40. What converts dUMP to dTMP? What drug blocks this?
AD; disorder of blood vessels that results in telangietasias - skin discolorations - and AVMs (arteriovenous malformations)
Thymidylate synthase; 5- Flourouracil
Mucus secreting goblet cells - ab secreting plasma cells - nt secreting neurons
Ile - Phe - Thr - Trp

41. What are some catalase positive bugs?
S. aureus - Aspergillus
Some viruses
Cells that stay in G0 and cannot replicate - instead they regenerate from stem cells; neurons - skeletal and cardiac muscles - and RBCs
Phenylethanolamine N- methyl transferase; SAM

42. What do enhancers/silencers bind?
acetyl coA
Inability to taste
Robertsonian translocations
TFs

43. From What aa does thyroxine come from?
Tyrosine; tryptophan
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)
Acetyl coA carboxylase converting Acetyl coA into malonyl coA; increased AMP - glucagon - epinephrine - palmitate; citrate
Tyrosine

44. fatty acid synthase requires ...
Avidin in raw egg whites prevents their reabsorption; also antibiotic use; dermatitis - alopecia - glossitis - lactic acidosis - enteritis
NADPH- reductive biosynthesis!
Calcium and calmodulin activate glycogen phosphorylase kinase
Highly condensed forms of DNA; mitosis; neoplasia - highly dividing and thus hyperchromatic

45. Which is the rate limiting enzyme in ethanol metabolism? Where is this enzyme located? Where is the other enzyme located? What is the limiting reagent?
Alcohol dehydrogenase; cytosol; mitochondria; NAD+
Endonucleases - exonucleases - DNA polymerase - DNA ligase
Tyrosine
Proprionyl coA; through methylmalonyl coA; B12

46. What ensures that during gluconeogenesis all the ATP wont be used in glycolysis as youre producing it?
Protein Kinase A inhibits pyruvate kinase also ATP and alanine have negative effects on pyruvate kinase
B12 and folate; B12; washed out
HMG CoA synthetase; HMG CoA reductase
HMG CoA reductase; converts HMG CoA to mevalonate

47. High blood levels of fructose - galactose - glucose can result in conversion to osmotically active alcohol forms by...
usually present late
aldose reductase
Branched alpha ketoacid dehydrogenase
tumor suppressor genes (p53 - Rb - BAX)

48. What happens outside the fibroblast to procollagen?
Asp and glu; negatively
succinyl coA (and then to OXA)
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
To increase serum Ca and phosphate for bone deposition

49. What are the clinical symptoms of NF1?
Convulsions - hyperirritiability - peripheral neuropathy - sideroblastic anemias; alcoholics and INH and fortified goats milk consumption
Either salvage by HGPRT and PRPP or conversion to xanthine and then uric acid by xanthine oxidase
Cafe au lait spots - optic pathway gliomas - lisch nodules - scoliosis - neural tumors
Lysine and arginine; basic

50. What disease is associated with chromosome 13?
Rb
Malabsorption problems (CF and sprue) and mineral oil intake
= q
Cafe au lait spots - optic pathway gliomas - lisch nodules - scoliosis - neural tumors