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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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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 can cause a def in vit D?
Renal failure - decreased sun - fat malabsorption - chronic liver disease - p450 induction - hypoparathyroidism (activates 1 alpha hyrdoxylase)
tubulin dimer
Orotic acid in blood and urine (from pyrimidine synthesis) - decreased BUN - and symptoms of hyperammonemia
Bone - Skin - Tendon - dentin - fascia - cornea - late wound repair

2. What is an ELISA? What are the two ways to perform it? What is the sensitive and specificity? give a relative clinical example.
Codons may encode same amino acid since there are only ~20 aa
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
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
Fructose bisphosphatase 2

3. How do you detect the different reasons that cause cobalamin def?
No
Schilling test
Renal calculi of uric acid (replaces uric acid in excretion)
Autosomal Dominant; NF1 and mit. diseases (variable expression) - ?

4. What type of aa is phenylalanine? What implications does this have on the presentation of phenylketonuria?
Tremor (asterixis) - slurring of speech - decrease temperature - somnolence - vomitting - cerebral edema - blurring of vision - neural focal deficits
3; 5; mit matrix; alpha ketoglutarate dehydrogenase complex
A topoisomerase II inhibitor; because cancer cells use II more so than healthy cells
Aromatic; musty body odor

5. What two substrates (other than pyruvate) does pyruvate carboxylase require?
A trinucleotide repeat disorder results in faulty methylation and thus expression of the FMR1 gene on the X chromosome (x linked recessive mode of inheritance)
ATP and biotin
Dihydropterin reductase; NADP+; phenylalanine to tyrosine and tyrosine to DOP
G6PD dehydrogenase; transketolases

6. other than Acetyl coA inhibiting pyruvate dehydrogenase complex - how else is it ensured that gluconeogenesis is occuring during FA oxidation and not glycolysis?
catecholamine synthesis (dopamine to norepinephrine)
Either salvage by HGPRT and PRPP or conversion to xanthine and then uric acid by xanthine oxidase
Acetyl coA activated pyruvate carboxylase in gluconeogenesis
Arylsulfatase A; cerebroside sulfate

7. What is the other name of B3? What is its function? Synthesis of B3 requires What other vitamin? What else?
3; 5; mit matrix; alpha ketoglutarate dehydrogenase complex
Glutamine PRPP amidotransferase
Tyrosinase
Niacin/Nicotinic acid; NAD+ and NADH used in redox reactions - and catabolic and anabolic reactions; B6; tryptophan

8. AD diseases are usually...
pleiotropic
It induces its action!
Nitric Oxide - creatinine - Urea
Silent - misssense - nonsense

9. What three enzymes lead to orotic acid accumulation? What reactions are they involved in?
Ortinithine transcarbamoylase - orotic acid phosphoribosyltransferase - and orotidine 5 phosphate decarboxylase; urea cycle; last two are involved in conversion of orotic acid to UMP (adding PRPP to orotic acid)
Thick mucus plugs lungs recurrent pulmonary infections (pseudomonas and aureus) - chronic bronchitis - bronchiectasis (which can lead to cor pulmonale!); biofilms form on resp tract
Smoking (FR takes up all the antioxidants); diets lacking fruit
Ehler Danlos; type III collagen def; joint dislocation - berry aneurysm - ecchymoses - organ rupture

10. What is the rate determining enzyme in HMP shunt?
Avidin in raw egg whites prevents their reabsorption; also antibiotic use; dermatitis - alopecia - glossitis - lactic acidosis - enteritis
Glucose 6 phosphate dehydrogenase
Intestine - renal - bone
Glycolysis - fatty acid synthesis - HMP shunt - protein synthesis (RER) - steroid synthesis (SER) - glycogenolysis - glycogenesis

11. Sporadic Retinoblastoma results is unilateral - but only familial is associated with...
osteosarcomas (Rb mutation is in all cells)
Hemolytic anemia; jaundice in newborn if mom gets too much vit K
= q
Urea cycle enzyme deficiency (mc: ornithine transcarbamoylase); liver disease - alcoholic cirrhosis and reye's

12. In What stage of the cell cycle must cells be in order to be analyzed for karyotyping? and From What fluids can you draw the cells? and for What is karyotyping is used for?
By turning it into fructose - but sorbitol first; glucose to sorbitol via aldose reductase and sorbitol to fructose sorbital dehydrogenase
Phenylethanolamine N- methyl transferase; SAM
Metaphase (all lined up); blood - bone marrow - amniotic fluid - placental tissue; used to diagnose chromosomal abnormalities (deletions - trisomies - sex chromosomal disorders)
No protein!

13. Which Hurlers is milder and presents with no corneal clouding?
Progressive neurodegeneration (spasticity d/t UMN disease) - NO hepatosplenomegaly - cherry red spot on macula - lysosomes with onion skin - muscle weakness
Phenylaline
Osteomalacia is vit D def seen in adults and causes bowed legs and results in increased fractures
XR - iduronate sulfatase

14. What is the impetus for ketoacidosis in starvation? alcoholism?
Mental retardation - rocker bottom feet - micrognathia (small jaw) - low set ears - clenched hands with polydactyly - congenital heart disease
Branching increases the rate of synthesis and breakdown of glycogen by having more sites to add on to and break off
Oxaloacetate depleted for gluconeogenesis; excess NADH shunts oxaloactetate towards malate
Mitochondrial inheritance

15. What converts dopamine to norepinephrine? using What cofactor?
Lysine and arginine; basic
Acetone
Dopamine beta hydroxylase; vit C
Cancers (rapidly synthesizing); MTX (blocks dihyrdofolate reductase); phenytoin (prevent conjugase from breaking down polyglutamate form to monoglutamate form; reabsorption in jejunum is blocked by alcohol and OCPs; also blocked by sulfonamides

16. On a 100 meter sprint What is used as energy? in general What is the rule with exercise? What is used in 1000 meter run?
Stored ATP - creatine phosphate - anaerobic glycolysis; as distances increase - ATP is obtained from additional resources; above plus ox phos
G6PD dehydrogenase; transketolases
CNS defects - mental retardation - death - feeding difficulties - vomiting - hypoglycemia; thiamine
Steroid - cholesterol FA synthesis; HMP shunt

17. What is ascorbic acid? What is its 5 main function?
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
Degeneration of retinal ganglion cells and axons; acute loss of central vision; mit. inheritance
= q
Osmotic diarrhea - bloating - cramps

18. What disease is associated with chromosome 7?
Cystic Fibrosis (CFTR gene)
Chronic pancreatitis leads to type 1 diabetes - pancreatic insufficiency leads to ADEK deficiencies - malabsorption and steatorrhea - blocked gallbladder ducts leads to secondary biliary cirrhosis
Cystine kidney stones (cystine staghorn calculi)
Aldose reductase in the lens converts built up galactose to galactitol Which is osmotically active

19. What type of disease is MERRF? What is the clinical presentation?
Inability to taste
Myoclonus epilepsy
N5N10 methylene THF
III

20. No offspring of affected males with a mit disease can...
show disease
Increased blood levels of ammonia - decreased BUN
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
kidney

21. What regulates Fructose 1 -6 bisphosphatase? What reaction is it found in?
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
P53 and Rb; p53 activates BAX Which inhibits BCL2 When there is DNA damage - Rb phosphorylation allows progression into S phase
Citrate is positive on it - AMP and fructose 2 -6 bisphosphate is negative on it; gluconeogenesis
Silent - misssense - nonsense

22. AD disease...
usually present late
Dihydrofolate reductase; vitamin B12; homocysteine
S adenosyl homocysteine; homocysteine transferase; B12 and THF; M- THF
Prevents neural tube defects in 1st trimester

23. What is the rate determining enzyme in TCA cycle?
FISH
Isocitrate dehydrogenase
Lactose in mothers milk contains galactose
Thick mucus plugs lungs recurrent pulmonary infections (pseudomonas and aureus) - chronic bronchitis - bronchiectasis (which can lead to cor pulmonale!); biofilms form on resp tract

24. What are five clinical features seen in Osteogenesis Imperfecta?
Thinned cornea from lack of connective tissue so see underlying choroidal lens
Pellagra
Multiple bone fractures - blue sclerae - hearing loss (abdnormal middle ear bones!) - dental imperfections - retarded wound healing
Peripheral neuropathy - developmental delay - optic atrophy - globoid cells

25. What step in pyrimidine synthesis requires aspartate?
Carbamoyl phosphate to orotic acid
No protein!
Reticulin - skin - blood vessels - uterus - fetal tissue - early phase of granulation tissue
Ceramide trihexoside

26. What is the first step in the respiratory burst/oxidative burst? in What disease is this deficient?
Carnitine acyltransferase I
Inosine/IMP; adenosine deaminase
NADPH oxidase; CGD
Increased nuchal translucency

27. What is the mcc of an autosomal trisomy?
Meiotic nondisjunction in mothers gametes during the first meiosis
are AR
Tryptophan
NF1

28. Fructose 2 -6 bisphophate is involved in...
allosteric regulation
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
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)
glutton

29. What is acyl coA dehydrogenase used in? What does a deficiency in this enzyme produce?
Mannose
Pulmonary - GI - reproductive (infertility in male due to absence of vas deferens and seminal fluid thickness)
pleiotropic
Fatty acid (acyl coA) oxidation to acetyl coA; increase in dicarboxylic acids - and decreased glucose and ketones

30. What carries 1 carbon units as energy?
Tetrahydrofolates
Either salvage by HGPRT and PRPP or converstion to xanthine and then to uric acid by xanthine oxidase
Silent - misssense - nonsense
liver

31. What are the clinical symptoms that result from I cell disease?
1) glycogenolysis (liver only contributes to plasma - muscle keeps it for itself) 2) gluconeogenesis
Glucagon - insulin and epinephrine
Arg
Coarse facial features - psychomotor retardation - clouded corneas - restricted joint movement - high plasma levels of lysosomal enzymes

32. Glucokinase has a LOW affinity (high Km) and...
All and Alzheimers
Dilated capillaries and veins; skin - mucous membranes - GI - resp - urinary tracts; rupture and can cause epistaxis - hemorrhage - GI bleeding
high Vmax
Uses HMG coA reductase

33. What is the metabolism of fructose?
osteosarcomas (Rb mutation is in all cells)
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
Fructose gets phosphorylated into fructose 1- P by fructokinase and then fructose 1P gets metabolized into DHAP or glyceraldehyde by aldolase B
Vimentin - desmin - cytokeratin - glial fibrillary acidic protein (GFAP) - neurofilaments; to differentiate between different cells

34. What is the deficient enzyme in Von Gierkes disease? What are the findings?
fastest sugar
Endonucleases are specific and break specific linking phosphodiester bonds - exonucleases are not sequence specific and just remove the nucleoside
Differentiation of cells - vision in reduced light (retinol makes up rhodopsin cells) - antioxidant
Severe fasting hypoglycemia - very high levels glycogen in liver - high blood lactate - hepatomegaly

35. What goes on with G0 cell cycle phase?
Cytoplasmic side; active
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
fatty acid degradation
NADPH oxidase - superoxide dismutase - myeloperoxidase; in the phagolysosome; HOCl (bleach); kills!

36. NAD+ is used in catabolic processes and NADPH is used in...
Dopa decarboxylase; B6
anabolic processes
Dactinomycin - doxorubicin
By cyclin CDK complexes

37. in Fabrys - What accumulates in tissue because fibroblasts express the def enzyme highly?
Ceramide trihexoside
2/3; LCAT (lecithin cholesterol acyltransferase)
Von gierkes
Limit dextrans

38. What is the RNA called before processing? and after?
HnRNA; mRNA
linked recessive
Lime and CaCO3 to displace the niacin
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

39. How is urea measured
A cytoskeletal protein - apart of flagella - cilia - mitotic spindles and transport (in neurons for example)
As BUN (blood urea nitrogen)
Branching increases the rate of synthesis and breakdown of glycogen by having more sites to add on to and break off
Mannose

40. How are fa oxidation and gluconeogenesis linked - ie What compound makes sure that they occur together and that glycolysis doesnt occur?
Alcoholism - RA - inflammatory disease - chronic diarrhea
Pyridoxine; converted to pyridoxal phsophate used in transamination reaction - decarboxylations - heme synthesis - niacin synthesis from trp - glycogen phosphorylase - cystathionine synthesis - NT synthesis
B hydroxybutyrate - acetone - acetoacetate; beta hydroxybutyrate; beta hydroxybutyrate
Acetyl coA and NADH inhibit pyruvate dehyrogenase

41. What converts dUMP to dTMP? What drug blocks this?
Galactokinase; galactose appears in blood and urine - sometimes infantile cataracts may present as failure to track objects or lack of a social smile
Osteoblasts; de phosphorylates pyrophosphate Which inhibits bone mineralization; bone mineralization
DiGeorge and Velocardiofacial; deletion on chromosome 22q11 results in aberrant dev of 3rd and 4th branchial pouches; digeorge has thymic and parathyroid prob - velocardiofacial has palate and facial instead
Thymidylate synthase; 5- Flourouracil

42. What are ragged red fibers?
teratogenic
D2 (ergocalciferol) - D3 (cholecalciferol) - D3 (cholecalciferol)
An allosteric regulator; fructose 1 phosphate; phosphofructokinase 2
Collections of abnormal mit collecting under sarcolemmal membranes and results in distortion of myofibrils

43. What disease is associated with chromosome 4?
Cafe au lait spots - optic pathway gliomas - lisch nodules - scoliosis - neural tumors
AD
Hungtington
glycogen

44. Name two diseases that have to do with elastin; discuss their pathogenesis.
Increased orotic acid in urine - megaloblastic anemia - +/- hyperammonemia
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
SnRNPs; RNA polymerase III
activation of PFK1

45. What happens in an aminotransferase reaction
S adenosyl homocysteine; homocysteine transferase; B12 and THF; M- THF
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
to ions
Fructose gets phosphorylated into fructose 1- P by fructokinase and then fructose 1P gets metabolized into DHAP or glyceraldehyde by aldolase B

46. Which intermediate filament is found connective tissue? muscle? epithelial cells? neuroglia? neurons? What can intermediate filaments be used for?
Chediak Higashi Syndrome; decreased phagocytosis results in recurrent pyogenic infections - partial albinism - and peripheral neuropathy
TPP (from B1- pyrophosphate) - FAD (B2) - NADH2 (B3) - CoA (B5 - panothenate) - Lipoic Acid
Vimentin - desmin - cytokeratin - glial fibrillary acidic protein (GFAP) - neurofilaments; to differentiate between different cells
cell cycle events

47. What are the symptoms of cri du chat syndrome? What is the pathogenesis?
Lactose (glucose and galactose)
Arginne
Bacterial synthesis in the intestine
Microcephaly - mental retardation - high pitched mewing/crying - epicanthal folds - cardiac abnormalities; microdeletion on short arm of chrom 5

48. What can cause a vit C deficiency?
usually present late
Smoking (FR takes up all the antioxidants); diets lacking fruit
Biotin (vit B7) and Vit K
A topoisomerase II inhibitor; because cancer cells use II more so than healthy cells

49. What contributes to the acidosis in alcoholism?
Neural tissue and adrenal medulla
Lactic acidosis and ketoacidosis
Nitric Oxide - creatinine - Urea
ATP and dATP

50. What carries methyl groups as energy?
Vit D levels
SAM
Multiple bone fractures - blue sclerae - hearing loss (abdnormal middle ear bones!) - dental imperfections - retarded wound healing
SAM; Vit C

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USMLE Step 1 Biochemistry

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