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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.
Don't refresh. All questions and answers are randomly picked and ordered every time you load a test.

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 are the symptoms involved in lactase deficiency?
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)
Dopamine beta hydroxylase; vit C
Osmotic diarrhea - bloating - cramps
HnRNA; mRNA

2. How is muscular dystrophies diagnosed?
Beta glucocerebrosidase; glucocerebroside
Lupus
Increased CPK and muscle biopsy
Inability to transport LCFA into mitochondria results in their build up and causes weakness - hypotonia And HYPOKETOTIC HYPOGLYCEMIA

3. What is elastin? How is it different from collagen? Where is it found? How is it broken down?
HMG CoA synthetase; HMG CoA reductase
A fibrous protein (ie produced by fibroblasts); it has lysine and proline but nonglycosylated; arteries - lungs - elastic ligaments - vocal cords - ligamenta flava (connect vertebrae); broken down by elastase Which is inhibited by alpha 1 antitrypsin
Arginine
Cleft palate - Abnormal faces - thymic aplasia (t cell def) - Cardiac defects - hypocalcemia (secondary to hypoparathyroid); 3rd and 4th branchial pouch aberrant development

4. What is imprinting?
B hydroxybutyrate - acetone - acetoacetate; beta hydroxybutyrate; beta hydroxybutyrate
When one allele on a gene is only expressed and the other is methylated/inactivated; during gametogenesis one of the alleles is methylated because the gamete from the other sex will provide those characteristics for the child
Multiple bone fractures - blue sclerae - hearing loss (abdnormal middle ear bones!) - dental imperfections - retarded wound healing
Cytoplasmic side; active

5. What is the inheritance of myoclonic epilepsy?
Lesch Nyhan syndrome; X linked recessive
Mitochondrial inheritance
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)
Gauchers; hepatosplenomegaly - aseptic necrosis of the femur - bone crises - Gauchers cells - crumpled tissue paper looking (fibrillar appearing macrophages in liver - spleen - b.m.)

6. polymerase chain reaction (PCR)- Name an application for it
Genotyping for mutant alleles (different sizes)
GALT
When there is glucagon; protein kinase A activation and thus phosphorylation of the complex resulting in the activation of fructose 2 -6 BPase and no PFK2
Fructose 1 -6 bisphosphatase

7. What is the inheritance of Tuberous Sclerosis?
AD
Schwann cells - lens - retina - kidneys because they only have aldose reductase and no sorbital dehydrogenase
Mannose
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

8. What are cilia made of ?
ATP depletion and highly aerobic tissues (heart and brain) are affected first; false - glucose infusion worsens
MT!
When different combination of exons get combined together; in Beta thallasemia
Lens and neural tissue; converts galactose to osmotically active galactitol

9. What experiment is chromosomes used for?
Neurologic defects - myopathy - lactic acidosis; giving ONLY ketogenic nutrient high fat content and aa (lysine and leucine)
Mcardles disease
Vincristine - vinblastine - and paclitaxel; the vinca alkaloids block formation of the spindle and paclitaxel prevents breakdown of the mT spindle
Karyotyping

10. Where does synthesis of longer chain fatty acids occur? What does it require?
SER and mitochondria; NADPH
GTP: gluconeogenesis - OXA to phosphoenolpyruvate
2 oxidative and nonoxidative; cytoplasm (both); none (none produced either - you are simply switching carriers of energy- from glucose 6 phosphate to NADPH)
Arginne

11. What type of vitamin is Vit D? Where do we get it? in What form?
Because carbamaoyl phosphate is involved in both urea cycle and pyrimidine synthesis
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)
In ability to reabsorb phosphate from renal tubular cells results in osteomalacia due to defective bone mineralization; no
When water is pulled in; polyols (sugar alcohols); cataracts - retinopathy - peripheral neuropathy all seen in chronic hyperglycemia in diabetes

12. 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?
Hemolytic anemia due to heinz bodies Which are oxidized hemoglobin precipitating within the RBCs
only yield acetyl coA
Dihydrofolate reductase; vitamin B12; homocysteine
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

13. What are three ketone bodies? Which is not detected in urine? Which do you see most in alcoholic ketoacidosis?
Hemolytic anemia with a blood smear with heinz bodies and bite cells (phagocytic removal of heinz bodies)
Chromosomal imbalance (downs for ex)
B hydroxybutyrate - acetone - acetoacetate; beta hydroxybutyrate; beta hydroxybutyrate
Ret

14. What is the reason for blue sclerae in Osteogenesis Imperfecta?
Thinned cornea from lack of connective tissue so see underlying choroidal lens
Liver; berry aneurysms (d/t hypertension!) - and mitral valve prolapse
Increased homocysteine in the urine - mental retardation - osteoporosis - marfanoid habitus - and atherosclerosis
Aromatic; musty body odor

15. What is a robertsonian translocation? What does it mean if its balanced?
Genotyping for mutant alleles (different sizes)
Highly condensed forms of DNA; mitosis; neoplasia - highly dividing and thus hyperchromatic
Cycloheximide
Its When two acrocentric chromosomes combine their long arms (and short arms disappear); When the translocation is functional

16. What is Maternal PKU?

17. What is dihydroxyphenylalanine ?
Depression - progressive dementia - choreiform movements; between the ages of 20 and 50
NF1
DOPA
Cartilage (including hyaline) - vitreous body - nucleus pulposus

18. CFTR channel ______ Cl into sweat - secretes NaCl in luminal secretions
reabsorbs
Glutamate
Via fructose 1 P by fructokinase in the liver; as glucose 1 by being GALT- ed ;)
Phospholipids - sphingolipids - glycolipids

19. What are the two different ways you can insert genes into transgenic mice for experiments?
Hydroxurea; deoxygenates ribose
Tryptophan
hexokinase - glucokinase (its a glutton)
Constitutive - random insertion of gene into mouse genome; conditional - targeted through homologous recombination

20. What are the three types of point mutations?
30S + 50S= 70S
Cysteine; tyrosine
Silent - misssense - nonsense
Edwards

21. What are the key intermediates in the TCA?
jaundice
Citrate - Isocitrate - alpha ketoglutarate - Succinyl CoA - Succinate - Fumarate - Malate - Oxaloacetate
Highly condensed forms of DNA; mitosis; neoplasia - highly dividing and thus hyperchromatic
Also have fair skin - blond hair - blue eyes - eczema

22. What is another way to trap glucose in cells but not by phosphorylating it? What enzymes are used?
Prolonged deficiency of Vitamin C results in decreased tensile strength of collagen because lack of hydroxylated lysines to crosslink (hydroxylation required vit C); bleeding gums - bone pain - poor wound healing - hemarthroses - glossitis - hemorrha
Mannose
FISH
By turning it into fructose - but sorbitol first; glucose to sorbitol via aldose reductase and sorbitol to fructose sorbital dehydrogenase

23. What two substrates (other than pyruvate) does pyruvate carboxylase require?
Cartilage (including hyaline) - vitreous body - nucleus pulposus
ATP and biotin
Glycogen synthase
Assist in spontaneous refolding of proteins

24. What are chromosomes? during What part of the cell cycle are they found? in What pathologic state would you see a lot of chromosomes?
Acetyl coA and NADH inhibit pyruvate dehyrogenase
Prolonged deficiency of Vitamin C results in decreased tensile strength of collagen because lack of hydroxylated lysines to crosslink (hydroxylation required vit C); bleeding gums - bone pain - poor wound healing - hemarthroses - glossitis - hemorrha
An allosteric regulator; fructose 1 phosphate; phosphofructokinase 2
Highly condensed forms of DNA; mitosis; neoplasia - highly dividing and thus hyperchromatic

25. How many RNA polymerases do prokaryotes have? What drug blocks prokaryotic RNA polymerase only? What is it used for?
1 for all three classes of RNA; Rifampin; TB
high Vmax
Hypophosphatemia rickets and Alports
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

26. Findings for Cori's disease? deficient enzyme?
2 -4 dinitrophenol - aspirin - and thermogenin (in brown fat)
Malnutrition - edema (hypoalbuminemia) - anemia liver (fatty change) - diarrhea d/t no brush border enzymes (no protein!)
Milder form of type 1 with normal blood lactate levels; debranching enzyme (alpha -1 -6 glucosidase)
enzymatic

27. What is the first enzyme and step in ethanol metabolism? second step? What is a cofactor in both of these steps? What vitamin does this come from?
Dilated capillaries and veins; skin - mucous membranes - GI - resp - urinary tracts; rupture and can cause epistaxis - hemorrhage - GI bleeding
Meconium ileus and failure to thrive; negative serum immunoreactive trypsin
Either salvage by HGPRT and PRPP or conversion to xanthine and then uric acid by xanthine oxidase
Alcohol dehyrogenase (alcohol to acetaldehyde); acetaldehyde dehydrogenase (acetaldehyde to acetate); NAD+; B3 (niacin)

28. What is this reaction dependent on?
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)
Oxygen!
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
Chronic pancreatitis leads to type 1 diabetes - pancreatic insufficiency leads to ADEK deficiencies - malabsorption and steatorrhea - blocked gallbladder ducts leads to secondary biliary cirrhosis

29. What are the mucopolysaccharidoses? Which one is XR? AR? What accumulates in them?
kidney
Pellagra
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
Hurlers Syndromes;Iduronate sulfatase deficiency; alpha L iduronidase def; heparan sulfate - dermatan sulfate

30. When are there low levels of cAMP? What does this result in?
False; GTP hydrolysis
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!!
COMT and MAO
glycogen

31. What is the precursor for pyrimidines?
Arg
(cofactor for 100+ enzymes!) needed for zinc fingers transcription motif - collagen synthesis - alcohol metabolism
Orotate
HMP shunt (to NADPH) and glycogenolysis (as glucose 1 phosphate)

32. From What aa do the catecholamines come from? and What aa does that aa come from?
It is apart of thiamine pyrophosphate Which is a cofactor enzymes involved in ATP production: pyruvate dehydrogenase (glycolysis) - alpha ketoglutarate dehydrogenase (TCA cycle) - transketolase (HMP shunt) - branched chain AA dehydrogenase
Cells that never got into G0 and divide rapidly with a short G1; bone marrow - gut epithelium - hair follicles - skin
Indirect inhibtion of the Na+/Ca+2 exchange resulting in increased intracellular calcium Which increases cardiac contractility
Tyrosine; phenylaline

33. What aa does melanin come from? melatonin?
Tyrosine; tryptophan
Because cells of mononuclear phagocytic are esp rich in lysosomes
D2 (ergocalciferol) - D3 (cholecalciferol) - D3 (cholecalciferol)
Glutamate and aspartate; CO2 provides an ketone

34. What does topoisomerase do? I or II eukaryotic?
XR - iduronate sulfatase
FA coA synthetase combines coA with FA to make acyl coA
Creates nicks in DNA to prevent supercoiling; both!
golgi apparatus

35. What type of drug are alkylating agents? How do they work? give a couple examples
Glutamine PRPP amidotransferase
Antineoplastic agents; alkylate DNA!; cyclophosphamide and nitrosurea (aka lomustrine - a nitrosource)
Imprinting Dx's: prader Willi and Angelmann
Arg - lys - his; positive except His has no charge at body pH

36. What is the mode of inheritance of Huntingtons?
Glycogen phosphorylase
By activating adenylyl cylase Which increases cAMP Which increases levels of protein kinase A Which activates glycogen phosphorylase kinase; epinephrine
AD (trinucleotide expansion is not a mode of inheritance)
Urine turns black on standing

37. What are two clinical symptoms of newborns with CF? and whats a lab value of a newborn with CF?
Cardiomegaly - cyanosis - tachycardia - dyspnea - vomitting; 2-3 months
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
A 16C FA! the product in FA synthesis; saturated; no double bonds
Meconium ileus and failure to thrive; negative serum immunoreactive trypsin

38. What are the findings with phenylketonuria?
Mental and growth retardation; seizures - fair skin - eczema - musty body odor; neurotoxic ketones and acids stimulate vomitting - can lead to pyloric stenosis
AR
Bacteria produce lactate acid and gases from lactose and produce acidic diarrhea and bloating
Schilling test

39. How many enzymes does the pyruvate dehydrogenase complex have? and How many cofactors? Where is it located? to What enzyme is it similar to?
3; 5; mit matrix; alpha ketoglutarate dehydrogenase complex
LDL; LDL oxidize atherosclerosis
Thymidylate synthase; 5- Flourouracil
Arg and his

40. What does Vit D stimulation of osteoblasts do?
severe
Tryptophan
adrenal medulla
Release alk phos

41. After addition of an aa - Where does the aminoacyl tRNA now bind to?
Pataus
30S ribosomal RNA (small one)
Sphingomyelinase; sphingomyelin
Milder form of type 1 with normal blood lactate levels; debranching enzyme (alpha -1 -6 glucosidase)

42. What is main lab finding in Lesch Nyhan syndrome? due to What enzyme deficiency? What reaction is deficient?
be anorexic
Shiga toxin and ricin (protein in castor beans)
Uric acidemia; HGPRT; purine salvage
Vincrstine (vinca alkaloids) and paclitaxel

43. If a cell has a hyperchromatic or condensed nucleus is it undergoing transcription etc?
activator
GTP: gluconeogenesis - OXA to phosphoenolpyruvate
Developmental delay - gargoylism - airway obstruction (d/t short neck) - corneal clouding - CAD - hepatosplenomegaly
No

44. Which amino acids makes up histones?
Phosphofructokinase 1; fructose 1 phosphate to fructose 1 -6 bisphosphate; enhancers: AMP - fructose 2 -6 Bisphosphate; downregulators: citrate - ATP
So that excess glucose can be stored (phosphorylated glucose cant leave) in the liver and saved as a buffer
Alpha amanitin; mushroom caps
Lys and arg

45. P450 induction decreases ___________ by increased metabolism
Zero order; increased production of NADH no matter the amount of end products produced
Vit D levels
tyrosine hydroxylase
30S ribosomal RNA (small one)

46. Which is the most common lysosomal storage disease? What are its findings?
Mitochondria - archaebacteria - mycoplasma - and some yeasts
Carnitine shuttle
Gauchers; hepatosplenomegaly - aseptic necrosis of the femur - bone crises - Gauchers cells - crumpled tissue paper looking (fibrillar appearing macrophages in liver - spleen - b.m.)
Hemolytic anemia; jaundice in newborn if mom gets too much vit K

47. What is a microarray and What is it used for?
Carbomyl phosphate synthetase I turns N acetyl glutamate into carbomyl phosphate and then ornithine transcarbomyalse turns carbomyl phosphate into citrulline (using ornithine)
all carriers
AR
A piece of glass or silicon chip with thousands of grids with different nucleic acid sequences and DNA or RNA probes are hybridized to the chip and a scanner detects the complementary binding; used to study a persons genome and to detect snps (single

48. From What aa does thyroxine come from?
Fructose 1 -6 bisphosphatase
Tyrosine
Uses HMG coA reductase
B hydroxybutyrate - acetone - acetoacetate; beta hydroxybutyrate; beta hydroxybutyrate

49. Do introns or exons contain the genetic information?
Lime and CaCO3 to displace the niacin
A cytoskeletal protein - apart of flagella - cilia - mitotic spindles and transport (in neurons for example)
Exons
Pellagra

50. What happens to homocysteine after converting a methyl?
Becomes methionine
Isocitrate dehydrogenase
pleiotropic
ATP