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USMLE Step 1 Biochemistry

Subjects : health-sciences, usmle-step-1

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Answer 50 questions in 15 minutes.
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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. Kwashiokor patients are not...
Iatrogenic - granulomatous disease (sarcoidosis d/t epitheloid macrophage release of calcitriol); hypercalcemia - hypercalciuria (renal calculi!) - loss of appetite - stupor
Phosphorylation of glucose to yield glucose 6 phosphate; ATP; first step of glycolysis
Phenylalanine build up
calorie deficient

2. Where does the HMP (pentose phosphate pathway) occur other than in the liver?
Lactating mammary glands - adrenal cortex (sites of fatty acid or steroid synthesis) - RBCS (for glutathione reduction for FR injury)
Neurologic defects - myopathy - lactic acidosis; giving ONLY ketogenic nutrient high fat content and aa (lysine and leucine)
Citrate shuttle in the inner mitochondrial membrane transports acetyl coA into the cytoplasm; liver; liver and adipose tissue
It induces its action!

3. Odd chain fatty acids yield one proprionyl coA which can enter the TCA cycle as...
Glycine; proline and lysine
Nitric Oxide - creatinine - Urea
succinyl coA (and then to OXA)
Reticulin - skin - blood vessels - uterus - fetal tissue - early phase of granulation tissue

4. What causes gout in alcoholism?
Nucleus
Abs against type IV collagen; pulmonary and glomerular capillaries
'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
Increased acid production competes with uric acid for excretion

5. What is the deficient enzyme in Krabbes disease? accumulated Substrate?
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
Arginine
Catalyze gamma carboxylation of glutamic acid residues on coag factors so they can bind to Ca complex
Galactocerebrosidase; Galactocerebroside

6. From What aa do NAD+ and NADP+ come from?
Both add phosphate; phosphorylase uses no ATP and inorganic phosphate
XR - iduronate sulfatase
Gets converted to glycerol 3 P Which is then converted to DHAP Which then can enter glycolyis
Tryptophan

7. What causes liver damage in kwashiokor?
AD
Aminoglycosides and tetracyclines
DNA dependent RNA polymerase
Increased TG synthesis from increased Glycerol 3 P and abetalipoproteinemia

8. ____________ disorders are worse than fructose disorders.
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
Bone - Skin - Tendon - dentin - fascia - cornea - late wound repair
Fructose gets phosphorylated into fructose 1- P by fructokinase and then fructose 1P gets metabolized into DHAP or glyceraldehyde by aldolase B
Galactose

9. What is the deficient enzyme in Niemann Pick disease? accumulated Substrate?
1) glycogenolysis (liver only contributes to plasma - muscle keeps it for itself) 2) gluconeogenesis
Sphingomyelinase; sphingomyelin
Oxaloacetate depleted for gluconeogenesis; excess NADH shunts oxaloactetate towards malate
Aldolase B; Fructose 1 phosphate accumulates Which uses up Phosphate and the result is inhibition of gluconeogenesis without phosphate; hypoglycemia - jaundice - cirrhosis - vomitting

10. Glucagon works on __________ to activate glycogenolysis by activating adenylyl cyclase
liver
Vincrstine (vinca alkaloids) and paclitaxel
Milder form of type 1 with normal blood lactate levels; debranching enzyme (alpha -1 -6 glucosidase)
Branche aa alpha ketoacid dehydrogenase; thiamine; Isoleucine - leucine - valine

11. What is carnitine deficiency and What does it result in?
NF1
HnRNA; mRNA
MT!
Inability to transport LCFA into mitochondria results in their build up and causes weakness - hypotonia And HYPOKETOTIC HYPOGLYCEMIA

12. What is the rate limiting step in glycolysis? What does it do? who activates it? inhibits it?
Pellagra (dermatitis - diarrhea - dementia); INH use - B6 deficiency - Hartnup disease - malignant carcinoid syndrome - and a corn based diet (lacks trp and niacin in corn cant be absorbed)
Lactating mammary glands - adrenal cortex (sites of fatty acid or steroid synthesis) - RBCS (for glutathione reduction for FR injury)
Phosphofructokinase 1; fructose 1 phosphate to fructose 1 -6 bisphosphate; enhancers: AMP - fructose 2 -6 Bisphosphate; downregulators: citrate - ATP
UV specific endonucleases for excision repair; AR; dry skin - photosensitivity - poikiloderma - susceptibility to skin cancers

13. Insulin receptor is not cAMP second receptor but rater tyrosine kinase...
Which dimerizes and activates a protein phosphatase Which de phosphorylates glycogen phosphorylase
Glucokinase
Acetyl coA carboxylase converting Acetyl coA into malonyl coA; increased AMP - glucagon - epinephrine - palmitate; citrate
30S + 50S= 70S

14. What cofactor is required of preprocollagen hydroxylation?
Nacetyl cysteine; breaks disulfide bridges of mucus plugs
Prevents neural tube defects in 1st trimester
Essential fructosuria and fructose intolerance; fructose intolerance
Vit C

15. What is the difference between rickets and osteomalacia?
Its the ribose; its made from PRPP synthetase using ribose 5 P from the HMP shunt
Osteomalacia is vit D def seen in adults and causes bowed legs and results in increased fractures
Biotin; rate limiting step in fA synthesis - acetyl coA into malonyl coA
Vit D and K

16. When are there low levels of cAMP? What does this result in?
A topoisomerase II inhibitor; because cancer cells use II more so than healthy cells
Steroid synthesis and detoxification
Where there is insulin; low protein kinase A means dePhosphorylation of the complex Which results in increased PFK2 Which results in more fructose 2 -6 bp and thus more PFK1!!
Succinate hydrogenase; matrix

17. What disease is associated with chromosome 13?
Pataus
Alcoholism - RA - inflammatory disease - chronic diarrhea
can be d/t lyonization
G6PD dehydrogenase; transketolases

18. What is the deficient enzyme in Von Gierkes disease? What are the findings?
DNA dependent RNA polymerase
Cycloheximide
Severe fasting hypoglycemia - very high levels glycogen in liver - high blood lactate - hepatomegaly
(cofactor for 100+ enzymes!) needed for zinc fingers transcription motif - collagen synthesis - alcohol metabolism

19. What is a common result of unbalanced robertsonian translocation?
Both add phosphate; phosphorylase uses no ATP and inorganic phosphate
Creates nicks in DNA to prevent supercoiling; both!
Kartaganer's aka ciliar dyskinesia; lack of dynein arm ATPase
Chromosomal imbalance (downs for ex)

20. Which aa are only ketogenic?
Lysine and leucine
tumor suppressor genes (p53 - Rb - BAX)
Degeneration of retinal ganglion cells and axons; acute loss of central vision; mit. inheritance
The products of FA oxidation (Acetyl CoA and NADH) phosphorylate pyruvate deyhdrogenase Which inactivates it

21. What creates a frameshift mutation? What are some clinical examples?
S - adenosyl methionine
Pellagra (dermatitis - diarrhea - dementia); INH use - B6 deficiency - Hartnup disease - malignant carcinoid syndrome - and a corn based diet (lacks trp and niacin in corn cant be absorbed)
Catalase positive because they reduce their H2O2 and WBCs cant use that to make bleach; because the way they protect is by building a wall around the organisms with granulomas
Insertion or deletion of a number of nucleotides indivisible by 3; tay sachs - duchennes MD

22. Which drug blocks the Na+K+ pump by blocking the K+ site?
Ouabain
B12 and folate; B12; washed out
LDL; LDL oxidize atherosclerosis
3; 5; mit matrix; alpha ketoglutarate dehydrogenase complex

23. What type exonuclease activity does DNA polymerase III have? in What organisms is it found?

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24. What is the deficient enzyme in Fabry's disease? accumulated Substrate?
Alpha galactosidase A; ceramide trihexoside
Cardiomegaly - cyanosis - tachycardia - dyspnea - vomitting; 2-3 months
Cells that stay in G0 and cannot replicate - instead they regenerate from stem cells; neurons - skeletal and cardiac muscles - and RBCs
Met - arg - val - his

25. Which cells have the most FA and thus are the most susceptible to FR injury?
CCA and chemically modified bases Where amino acid is covalently bonded
Neurons and RBCs
Prevents neural tube defects in 1st trimester
Lesch Nyhan syndrome; X linked recessive

26. 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?
Type IV collagen defect; X linked recessive; hereditary glomerulonephritis - deafness - ocular disturbances
Macrocytic megaloblastic anemia; it is the most common vit def in the US; no neuro symptoms and no methylmalonyl coA in the urine
Back up of pyruvate and alanine and thus lactic acidosis occurs; can be d/t alcoholism (vit B1 def)
Dopamine

27. How do WBCs of CGD patients do some damage?
Glutamate
No protein!
kidney
They use the H2O2 produced by some bacteria to produce bleach with myeloperoxidase Which they still have

28. What is the product of alpha ketoglutarate dehdrogenase?
Succinyl coA
5' end; 3' end
Nucleus
Fatty acid (acyl coA) oxidation to acetyl coA; increase in dicarboxylic acids - and decreased glucose and ketones

29. What enzyme converts DHF to THF? What vitamin cofactor is used to transfer a methyl from an amino acid to DHF? What amino acid is that?
Urea cycle enzyme deficiency (mc: ornithine transcarbamoylase); liver disease - alcoholic cirrhosis and reye's
Flouroquinolones
homocysteine transferase
Dihydrofolate reductase; vitamin B12; homocysteine

30. No offspring of affected males with a mit disease can...
Neurons and RBCs
show disease
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
By epoxide reductase in the liver; coumadin; intestinal flora and leafy vegetables

31. How can hyperammonia be hereditary? acquired?

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32. For What three enzymes is arginine a precursor for?
Steroid - cholesterol FA synthesis; HMP shunt
Nitric Oxide - creatinine - Urea
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)
NADPH- reductive biosynthesis!

33. Other than glycogenesis and glycolysis - What else is Fructose used for?
Mannose
Mitochondrial inheritance
32 or 30; malate aspartate shuttle or glycerol 3 phosphate shuttle; heart/liver and muscle
fructose

34. What is uniparental disomy? give an example of a disease it can cause.
Hexosaminidase A; GM2 ganglioside
B hydroxybutyrate - acetone - acetoacetate; beta hydroxybutyrate; beta hydroxybutyrate
Alcohol dehydrogenase; cytosol; mitochondria; NAD+
When child gets two chromosomes from one parent; imprinting dx

35. How many calories does one gram of protein generate? carb? fat?
4 -4 -9
Fructokinase; benign - fructose in the blood and urine
(cofactor for 100+ enzymes!) needed for zinc fingers transcription motif - collagen synthesis - alcohol metabolism
An allosteric regulator; fructose 1 phosphate; phosphofructokinase 2

36. What disease is associated with chromosome 15?

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37. What two substrates (other than pyruvate) does pyruvate carboxylase require?
ATP and biotin
H1
Actin/myosin - MT - intermediate filaments
Prolonged Ab use

38. What is the mode of inheritance of Marfans?
Leucine and lysine
Cells that stay in G0 and cannot replicate - instead they regenerate from stem cells; neurons - skeletal and cardiac muscles - and RBCs
AD
Beta thalassemia

39. in What organ is D3 converted from 25 OH D3? with What enzymes?
Coenzyme A and lipoamides
Osteoblasts; de phosphorylates pyrophosphate Which inhibits bone mineralization; bone mineralization
Liver; P450 hydroxylation
Marfans and emphysema; marfans is a fibrillin protein defect - fibrillin is a supporting protein in tropoelastin scaffolding and results in aorta - skeletal and eye defects; emphysema is due to a deficiency in the function of alpha 1 antitrypsin

40. Three causes of albinism. What does albinism increased risk of?
Tyrosinase deficiency; defective tyrosine transporters; lack of migration of neural crest cells; skin cancer
Oxygen!
both liver and muscle (unlike glucagon)
Cystine kidney stones (cystine staghorn calculi)

41. What is type II collagen found?
Assist in spontaneous refolding of proteins
Kidney and intestinal epithelium
golgi apparatus
Cartilage (including hyaline) - vitreous body - nucleus pulposus

42. What metabolic reactions occur in the cytoplasm?
Prevents neural tube defects in 1st trimester
3'--> 5'; prokaryote only
kidney
Glycolysis - fatty acid synthesis - HMP shunt - protein synthesis (RER) - steroid synthesis (SER) - glycogenolysis - glycogenesis

43. In What tissues can sorbitol accumulate? why?
B hydroxybutyrate - acetone - acetoacetate; beta hydroxybutyrate; beta hydroxybutyrate
Homocysteine methyltransferase - cystathionine reductase - decreased affinity of cystathionine reductase for pyridoxal phosphate; AR
Urine turns black on standing
Schwann cells - lens - retina - kidneys because they only have aldose reductase and no sorbital dehydrogenase

44. What kind of DNA damage does ionizing radiation cause? What type of repair helps with this?
AD
Double strand breaks; nonhomologous end joining
Pernicious anemia; older - mentally slow european woman who is anemic and icteric with a smooth shiny tongue and has broad based shuffling gait
Macrocytic megaloblastic anemia - hypersegmented PMNs - subacute combined degeneration myelopathy Which results in degeneration of the dorsal and lateral columns (loss of position and vibration sense - spastic paresis and ataxia) due to abnormal myel

45. From What aa does serotonin come from? What NT is serotonin a precursor for?
MT!
Tryptophan; melatonin
An anti cancer agent that is a topoisomerase II and creates nicks in DNA
NAD+; through ETC you produce NAD+; through production of Lactate from pyruvate you create lactate

46. How is urea measured
catecholamine synthesis (dopamine to norepinephrine)
Chronic pancreatitis leads to type 1 diabetes - pancreatic insufficiency leads to ADEK deficiencies - malabsorption and steatorrhea - blocked gallbladder ducts leads to secondary biliary cirrhosis
Isocitrate dehydrogenase
As BUN (blood urea nitrogen)

47. What is the inheritance pattern of hereditary hemorrhagic telangiectasia or Olser Weber Rendu Syndrome? What is it?
Heteroplasmy
Asp and glu; negatively
AD; disorder of blood vessels that results in telangietasias - skin discolorations - and AVMs (arteriovenous malformations)
When the 2nd hit of the allele is What causes the disease; sporadic retinoblastoma

48. How is CF treated? How does it work?
When the amino acid is covalently binded to its corresponding tRNA; Aminoacyl tRNA synthetase; ATP; translation
Uses HMG coA reductase
Nacetyl cysteine; breaks disulfide bridges of mucus plugs
P53 and Rb; p53 activates BAX Which inhibits BCL2 When there is DNA damage - Rb phosphorylation allows progression into S phase

49. What is the presentation of G6PD deficiency?
Hemolytic anemia with a blood smear with heinz bodies and bite cells (phagocytic removal of heinz bodies)
When child gets two chromosomes from one parent; imprinting dx
RBCs - renal medulla - leukocytes - lens - testes - cornea
Its When two acrocentric chromosomes combine their long arms (and short arms disappear); When the translocation is functional

50. The __________ adds O linked oligosaccharides to serine and threonine residues on proteins
Cri du Chat
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
Griseofulvin
golgi apparatus

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USMLE Step 1 Biochemistry

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