Biochemistry USMLE Step 1 Questions Answered Correctly Latest Version 2024-2025
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.
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