Biochemistry USMLE Step 1 Questions Answered Correctly Latest Version

Biochemistry USMLE Step 1 Questions Answered Correctly Latest Version Achondroplasia involves what defect? - Answers Cell-signlaling defect of fibroblast growth factor (FGF) receptor 3 Marfan's syndrome involves a mutation in what? - Answers Fibrillin gene Nucleoside + phosphate= ? - Answers Nucleotide Adenine and Guanine are in what type of nucleic acid? - Answers Purines Cytosine, Uracil and Thyamine arein what kind of nucleic acid? - Answers Pyrimidines Xanthine, hypoxanthine and uric acid are all what type of base? - Answers Purine How many hydrogen bonds are found between Guanine and Cytosine? - Answers 3 Chargaff's ruledictates that the percent of adenine in a DNA molecule is = to the % of _____ in a DNA molecule. - Answers thyamine What is the rate limiting step in Purine de novo synthesis? - Answers PRPP step via glutamin PRPP amidotranferase PRPP amidotransferase inhibited by what downstream products in purine synthesis? - Answers IMP, GMP, AMP and puring analogs Allopurinola and 6-mercaptopurine What are the required cofactors for purine de novo synthesis? - Answers THF, glutamine, glycine, aspartate, CO2 Name two crucial enzymes involved in purine salvage pathways - Answers hypoxanthine Phosoribosyltransferaase (HGPRT) anf Adenosine Deaminase. HGPRT converts guanine to ______ and hypoxanthine to _______. - Answers GMP; IMP Allopurinol and 6-mercaptopurine are purine analogs that inhibit ________. - Answers PRPP amidotransferase Allopurinol is used for the treatment of chronic gout because it __________. - Answers Decreases uric acid levels by blocking xanthine oxidase with converts hypoxanthing to xanthine and then to uric acid Lesch-Nyhan is a X-lined recessive disorder due to __________. - Answers absence of HGPRT--thus patients are unable to salvage purines leading to excessive de novov synthesis which then leads to excess uric acid production. The presentation of retardation, cerebral palsy, slef-mutiliation, gout and "crystals in diaper" and self-muilation is most consistent with ________. - Answers Lesch-Nyahan Syndrome What is the treatment for Lesch-Nyhan syndrome? - Answers Allopurinol (inhibits xanthine oxidase in the purine salvage pathway, thus reducing excesss production of uric acid) What disorder is associated with hyperuricenmis either due to overproduction or underexcretion of uric acis and is charcterized by sodium urate crystal deposits in the joints and soft tissues? - Answers Gout What are some causes of uric acid overproduction in aquired gout? - Answers Leukemia, alcoholism, myelopproliferative syndrom, multiple myeloma, hemolysis, neoplasia, psoriasis What drugs are associated with urate underexcretion in aquired gout? - Answers Aspirin, diuretics, alcohol What is podagra? - Answers Gout of the great toe Monoarticular arthirits of distal joints often with a history of hyperuricemia percipiatated by a sudden change in urate levels (due to large meals, alcohol) is characteristic of ______ and can eventually lead to the formation of _______. - Answers gout; nodular tophi (urate crystals surrounded by fibrous connective tissue) located around the joints and Achilles tendon. How is gout diagnosed? - Answers Detection of negatively bifringent crystals from articular tap, arthritis and hyperuricemia What is the treatment for Gout? - Answers For chronic-allopurinol to lower uric acid levels but NOT for acute treatment. For aute treatment decrease pain and inflammation with colchicines and NSAIDS and avoidance of large meals and ETOH What is colchicine used for and how does it work? - Answers Acute Gout. It prevents neutrophil and leukocyte migration into the joint by binding tubulin, depolarizing microtubules and interefering with motility and degranulation. Note: Indomethacin is more commonly used due to less side effects SCID (Severe Combined T and B Immunodeficieny) is an autsomal recessive disorder cause by a deficiency in what enzyme? - Answers Adenosine deaminse How does a deficeincy in adenosine deaminase cause SCID? - Answers W/ a deficiency in adenosidne deaminase (impt enzyme involved purine synthesis) causes the build up of ATP and dATP which in turn inhibit ribonucleotide reductase thus preventing DNA synthesis and decreasing lymphocyte count. Children with no plasma cells or B or T lymphocytes on CBC and no thymus and recurrent infections (esp. Candida and Pneumocystis jiroveci) are likely to have _______. - Answers SCID What is the treatment for SCID? - Answers Gene therapy and Bone marrow transplant What chemotherapy drug inhibit ribonucleotide reductase (enzyme that converts NTP TO dNTPS)? - Answers hydroxyurea Hereditary Orotic Aciduria is due to ______________. - Answers The inability to convert orotic acid to UMP (de novo pyrimidine synthesis pathway) and thus orotic acid builds up in urine. Orotic acid in urine, megaloblastic anemia that is not corrected with B12 or folic acid and no hyperammonemia is consistent with _________. - Answers Orotic Aciduria Orotic acid in urine, no megaloblastic anemia and hyperammonemia is consistent with _________. - Answers OTC (Orthinine Transcarboxylase) deficiency-- missing enzyme cause carbomyl phosphate to be shunted from Urea cycle to the pyrmidine synthesis pathway and get an excess or orortic acid in addition to reduced excretion of urea thus elevated ammonia levels. Trimethoprim + Sulfamethoxazole (aka Bactrim) works to sequentially block ______ sysnthesis. - Answers Folate ___-DNA is right handed while ____-DNA is left handed - Answers right; left What enzyme is responsible for nicking the DNA and realse the physical strain of the helix during replication? - Answers Topoismerases What type of Antibiotics inhibit bacterial topoisomerase 4? - Answers Quinolones _____chormatin is tightly coiled and transcriptionally inactive while _____chromatin is less condensed and is transcriptionally active. - Answers Heterochromatin; Euchromatin DNA methylation often ______ genes while histone acetylation ______. - Answers Inactivates (decreases gene transcription); Loosens DNA making it more accessible and more easily transcribed. Sjogren's syndrome (lymphocytic infiltration of salvivary and lacrimal glands resulting in dry mouth/dryeyes) is often associate with antibodies against SS-A/B (single strand binding proteins). What role do these proteins play in prokaryotic DNA replication? - Answers SS-DNA binding protein bind and stabilize ssdna to prevent re-annealing DNA polymerase III has polymerase activity in the _______ direction and exonuclease activity in the _______ direction. - Answers 5 to 3; 3 to 5 The primer for DNA replication synthesized by primase is made of DNA or RNA? - Answers RNA RNA primer removal in DNA synthesis is done by dna polymerase _____. - Answers I What DNA replication enzyme is responsible for sealing the remaining nicks? - Answers DNA ligase What are the 3 stop codons? - Answers UGA, UAG, UAA