BIOCHEMISTRY USMLE STEP 1 EXAM
2025 QUESTIONS AND ANSWERS
Achondroplasia involves what defect? - ANS Cell-signlaling defect of fibroblast growth factor
(FGF) receptor 3
Marfan's syndrome involves a mutation in what? - ANS Fibrillin gene
Nucleoside + phosphate= ? - ANS Nucleotide
Adenine and Guanine are in what type of nucleic acid? - ANS Purines
Cytosine, Uracil and Thyamine arein what kind of nucleic acid? - ANS Pyrimidines
Xanthine, hypoxanthine and uric acid are all what type of base? - ANS Purine
How many hydrogen bonds are found between Guanine and Cytosine? - ANS 3
Chargaff's ruledictates that the percent of adenine in a DNA molecule is = to the % of _____ in a
DNA molecule. - ANS thyamine
What is the rate limiting step in Purine de novo synthesis? - ANS PRPP step via glutamin PRPP
amidotranferase
1 @COPYRIGHT THEBRIGHT 2025/2026
,PRPP amidotransferase inhibited by what downstream products in purine synthesis? -
ANS IMP, GMP, AMP and puring analogs Allopurinola and 6-mercaptopurine
What are the required cofactors for purine de novo synthesis? - ANS THF, glutamine, glycine,
aspartate, CO2
Name two crucial enzymes involved in purine salvage pathways - ANS hypoxanthine
Phosoribosyltransferaase (HGPRT) anf Adenosine Deaminase.
HGPRT converts guanine to ______ and hypoxanthine to _______. - ANS GMP; IMP
Allopurinol and 6-mercaptopurine are purine analogs that inhibit ________. - ANS PRPP
amidotransferase
Allopurinol is used for the treatment of chronic gout because it __________. - ANS 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 __________. - ANS 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 ________. - ANS Lesch-Nyahan Syndrome
What is the treatment for Lesch-Nyhan syndrome? - ANS Allopurinol (inhibits xanthine
oxidase in the purine salvage pathway, thus reducing excesss production of uric acid)
2 @COPYRIGHT THEBRIGHT 2025/2026
, 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? - ANS Gout
What are some causes of uric acid overproduction in aquired gout? - ANS Leukemia,
alcoholism, myelopproliferative syndrom, multiple myeloma, hemolysis, neoplasia, psoriasis
What drugs are associated with urate underexcretion in aquired gout? - ANS Aspirin,
diuretics, alcohol
What is podagra? - ANS 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 _______. - ANS gout; nodular tophi (urate crystals
surrounded by fibrous connective tissue) located around the joints and Achilles tendon.
How is gout diagnosed? - ANS Detection of negatively bifringent crystals from articular tap,
arthritis and hyperuricemia
What is the treatment for Gout? - ANS 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? - ANS 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? - ANS Adenosine deaminse
3 @COPYRIGHT THEBRIGHT 2025/2026
2025 QUESTIONS AND ANSWERS
Achondroplasia involves what defect? - ANS Cell-signlaling defect of fibroblast growth factor
(FGF) receptor 3
Marfan's syndrome involves a mutation in what? - ANS Fibrillin gene
Nucleoside + phosphate= ? - ANS Nucleotide
Adenine and Guanine are in what type of nucleic acid? - ANS Purines
Cytosine, Uracil and Thyamine arein what kind of nucleic acid? - ANS Pyrimidines
Xanthine, hypoxanthine and uric acid are all what type of base? - ANS Purine
How many hydrogen bonds are found between Guanine and Cytosine? - ANS 3
Chargaff's ruledictates that the percent of adenine in a DNA molecule is = to the % of _____ in a
DNA molecule. - ANS thyamine
What is the rate limiting step in Purine de novo synthesis? - ANS PRPP step via glutamin PRPP
amidotranferase
1 @COPYRIGHT THEBRIGHT 2025/2026
,PRPP amidotransferase inhibited by what downstream products in purine synthesis? -
ANS IMP, GMP, AMP and puring analogs Allopurinola and 6-mercaptopurine
What are the required cofactors for purine de novo synthesis? - ANS THF, glutamine, glycine,
aspartate, CO2
Name two crucial enzymes involved in purine salvage pathways - ANS hypoxanthine
Phosoribosyltransferaase (HGPRT) anf Adenosine Deaminase.
HGPRT converts guanine to ______ and hypoxanthine to _______. - ANS GMP; IMP
Allopurinol and 6-mercaptopurine are purine analogs that inhibit ________. - ANS PRPP
amidotransferase
Allopurinol is used for the treatment of chronic gout because it __________. - ANS 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 __________. - ANS 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 ________. - ANS Lesch-Nyahan Syndrome
What is the treatment for Lesch-Nyhan syndrome? - ANS Allopurinol (inhibits xanthine
oxidase in the purine salvage pathway, thus reducing excesss production of uric acid)
2 @COPYRIGHT THEBRIGHT 2025/2026
, 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? - ANS Gout
What are some causes of uric acid overproduction in aquired gout? - ANS Leukemia,
alcoholism, myelopproliferative syndrom, multiple myeloma, hemolysis, neoplasia, psoriasis
What drugs are associated with urate underexcretion in aquired gout? - ANS Aspirin,
diuretics, alcohol
What is podagra? - ANS 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 _______. - ANS gout; nodular tophi (urate crystals
surrounded by fibrous connective tissue) located around the joints and Achilles tendon.
How is gout diagnosed? - ANS Detection of negatively bifringent crystals from articular tap,
arthritis and hyperuricemia
What is the treatment for Gout? - ANS 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? - ANS 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? - ANS Adenosine deaminse
3 @COPYRIGHT THEBRIGHT 2025/2026
