1 Exampromax - Stuvia US 2025/2026
USMLE STEP 1: First Aid-
Diseases/Disorders/Deficiencies/Syndromes Questions
with Detailed Verified Answers (100% Correct Answers)
/Already Graded A+
Adenosine deaminase (ADA) deficiency
Ans: An immunodeficiency disorder and one type of SCIDs that is caused
by an *inborn error in the metabolism of adenine*. The accumulation of
adenine destroys both B and T lymphocytes.
Increase in dATP is lymphotoxic
Lesch-Nyhan syndrome
Exampromax - Stuvia US
Ans: *Defective purine salvage due to absent HGPRT*, which converts
Approved By:
hypoxanthine to IMP and guanine to GMP. Results in *excess uric acid
production and de novo purine synthesis*.
X-linked recessive
Findings: intellectual disability, self-mutilation, aggression, hyperuricemia
(orange "sand" [sodium urate crystals] in diaper), gout, dystonia.
Treatment: allopurinol or febuxostat (2nd line)
Bloom Syndrome
Ans: Caused by mutations in the BLM gene leading to *mutated DNA
helicase protein formation*
Rare autosomal recessive disorder characterized by short stature,
predisposition to the development of cancer and genomic instability
Xeroderma pigmentosum (XP)
, 2 Exampromax - Stuvia US 2025/2026
Ans: *Nucleotide Excision Repair Defect*
Defect prevents repair of pyrimidine dimers because of ultraviolet light
exposure
Lynch Syndrome (Hereditary Nonpolyposis Colorectal Cancer)
Ans: Mismatch repair defect
Ataxia Telangiectasia
Ans: Nonhomologous end joining defect
*Defects in ATM gene = failure to repair DNA double strand breaks cell
Exampromax - Stuvia US
cycle arrest.*
Approved By:
Triad: *cerebellar defects (Ataxia), spider Angiomas (telangiectasia), IgA
deficiency.*
Increased AFP.
Decreased IgA, IgG, and IgE.
Lymphopenia, cerebellar atrophy.
Fanconi anemia
Ans: Nonhomologous end joining defect
I-Cell Disease (Inclusion cell disease/mucolipidosis type II)
, 3 Exampromax - Stuvia US 2025/2026
Ans: Inherited *lysosomal storage disorder*
*Defect in N-acetylglucosaminyl-1-phosphotransferase = failure of the
Golgi to phosphorylate mannose residues (ie, mannose-6-phosphate)* on
glycoproteins = proteins are secreted extracellularly rather than delivered to
lysosomes.
Results in coarse facial features, clouded corneas, restricted joint movement,
and high plasma levels of lysosomal enzymes.
Often fatal in childhood
Zellweger syndrome
Exampromax - Stuvia US
Ans: Autosomal Recessive disorder of *Peroxisome* biogenesis due to
Approved By:
mutated PEX genes
Results in hypotonia, seizures, hepatomegaly, early death
Refsum disease
Ans: Autosomal recessive disorder of *alpha oxidation* *(peroxisome)* =
*phytanic acid not metabolized to pristanic acid*
Results in scaly skin, ataxia, cataracts/night blindness, shortening of 4th
toe, epiphyseal dysplasia
Tx: diet, plasmaphoresis
Adrenoleukodystrophy (ALD)
, 4 Exampromax - Stuvia US 2025/2026
Ans: an X-linked disorder of *Beta oxidation (peroxisome)* in which
dysfunction of the adrenal cortex and nervouse system demyelination are
associated with high levels of saturated very-long-chain fatty acids
(VLCFAs)
Results in *VLCFAs buildup in adrenal glands, white matter of the brain,
testes*
Progressive disease that can lead to adrenal gland crisis, coma, death
Kartagener syndrome (primary ciliary dyskinesia)
Exampromax - Stuvia US
Ans: *Immotile cilia due to a dynein arm defect.*
Approved By:
Results in male and female infertility due to immotile sperm and
dysfunctional fallopian tube cilia, respectively; increased risk of ectopic
pregnancy.
Can cause bronchiectasis, recurrent sinusitis, and situs inversus (eg,
dextrocardia on CXR = Defect in left-right Dynein can lead to
Dextrocardia)
Osteogenesis Imperfecta
USMLE STEP 1: First Aid-
Diseases/Disorders/Deficiencies/Syndromes Questions
with Detailed Verified Answers (100% Correct Answers)
/Already Graded A+
Adenosine deaminase (ADA) deficiency
Ans: An immunodeficiency disorder and one type of SCIDs that is caused
by an *inborn error in the metabolism of adenine*. The accumulation of
adenine destroys both B and T lymphocytes.
Increase in dATP is lymphotoxic
Lesch-Nyhan syndrome
Exampromax - Stuvia US
Ans: *Defective purine salvage due to absent HGPRT*, which converts
Approved By:
hypoxanthine to IMP and guanine to GMP. Results in *excess uric acid
production and de novo purine synthesis*.
X-linked recessive
Findings: intellectual disability, self-mutilation, aggression, hyperuricemia
(orange "sand" [sodium urate crystals] in diaper), gout, dystonia.
Treatment: allopurinol or febuxostat (2nd line)
Bloom Syndrome
Ans: Caused by mutations in the BLM gene leading to *mutated DNA
helicase protein formation*
Rare autosomal recessive disorder characterized by short stature,
predisposition to the development of cancer and genomic instability
Xeroderma pigmentosum (XP)
, 2 Exampromax - Stuvia US 2025/2026
Ans: *Nucleotide Excision Repair Defect*
Defect prevents repair of pyrimidine dimers because of ultraviolet light
exposure
Lynch Syndrome (Hereditary Nonpolyposis Colorectal Cancer)
Ans: Mismatch repair defect
Ataxia Telangiectasia
Ans: Nonhomologous end joining defect
*Defects in ATM gene = failure to repair DNA double strand breaks cell
Exampromax - Stuvia US
cycle arrest.*
Approved By:
Triad: *cerebellar defects (Ataxia), spider Angiomas (telangiectasia), IgA
deficiency.*
Increased AFP.
Decreased IgA, IgG, and IgE.
Lymphopenia, cerebellar atrophy.
Fanconi anemia
Ans: Nonhomologous end joining defect
I-Cell Disease (Inclusion cell disease/mucolipidosis type II)
, 3 Exampromax - Stuvia US 2025/2026
Ans: Inherited *lysosomal storage disorder*
*Defect in N-acetylglucosaminyl-1-phosphotransferase = failure of the
Golgi to phosphorylate mannose residues (ie, mannose-6-phosphate)* on
glycoproteins = proteins are secreted extracellularly rather than delivered to
lysosomes.
Results in coarse facial features, clouded corneas, restricted joint movement,
and high plasma levels of lysosomal enzymes.
Often fatal in childhood
Zellweger syndrome
Exampromax - Stuvia US
Ans: Autosomal Recessive disorder of *Peroxisome* biogenesis due to
Approved By:
mutated PEX genes
Results in hypotonia, seizures, hepatomegaly, early death
Refsum disease
Ans: Autosomal recessive disorder of *alpha oxidation* *(peroxisome)* =
*phytanic acid not metabolized to pristanic acid*
Results in scaly skin, ataxia, cataracts/night blindness, shortening of 4th
toe, epiphyseal dysplasia
Tx: diet, plasmaphoresis
Adrenoleukodystrophy (ALD)
, 4 Exampromax - Stuvia US 2025/2026
Ans: an X-linked disorder of *Beta oxidation (peroxisome)* in which
dysfunction of the adrenal cortex and nervouse system demyelination are
associated with high levels of saturated very-long-chain fatty acids
(VLCFAs)
Results in *VLCFAs buildup in adrenal glands, white matter of the brain,
testes*
Progressive disease that can lead to adrenal gland crisis, coma, death
Kartagener syndrome (primary ciliary dyskinesia)
Exampromax - Stuvia US
Ans: *Immotile cilia due to a dynein arm defect.*
Approved By:
Results in male and female infertility due to immotile sperm and
dysfunctional fallopian tube cilia, respectively; increased risk of ectopic
pregnancy.
Can cause bronchiectasis, recurrent sinusitis, and situs inversus (eg,
dextrocardia on CXR = Defect in left-right Dynein can lead to
Dextrocardia)
Osteogenesis Imperfecta
