USMLE Step 1 Quiz
Practice Molecular Genetics And Gene Expression in USMLE Step 1 with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.
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A 12-year-old girl has short stature, webbed neck, and widely spaced nipples. She has primary amenorrhea. Echocardiogram shows coarctation of the aorta. Karyotype analysis reveals 45,X. The clinician explains that haploinsufficiency of genes that escape X-inactivation, such as SHOX, alters transcriptional programs for skeletal growth. Which of the following is most likely disrupted in this patient?
This quiz focuses on Molecular Genetics And Gene Expression, giving you a quick way to practice the rules, question types, and explanations that matter most for USMLE Step 1.
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A 12-year-old girl has short stature, webbed neck, and widely spaced nipples. She has primary amenorrhea. Echocardiogram shows coarctation of the aorta. Karyotype analysis reveals 45,X. The clinician explains that haploinsufficiency of genes that escape X-inactivation, such as SHOX, alters transcriptional programs for skeletal growth. Which of the following is most likely disrupted in this patient?
Explanation: This question assesses understanding of molecular genetics and gene expression, focusing on dosage effects in sex chromosome disorders. Gene expression from X-linked genes escaping inactivation, like SHOX, influences skeletal growth when dosage is reduced. In this vignette, the girl's short stature, webbed neck, and 45,X karyotype indicate Turner syndrome, with haploinsufficiency altering growth transcription. The correct answer identifies reduced dosage of escaping genes, impairing skeletal programs. Choice B is incorrect as it describes Down syndrome, unrelated to the cardiac coarctation. Encourage students to interpret karyotypes and monitor for comorbidities. Practice distinguishing aneuploidy phenotypes by molecular mechanisms.
A 42-year-old man presents with episodic headaches, diaphoresis, and hypertension. Plasma metanephrines are elevated. CT shows an adrenal mass consistent with pheochromocytoma. Family history reveals his mother had medullary thyroid carcinoma. Exam shows a thyroid nodule. Karyotype is 46,XY. Germline testing identifies a heterozygous activating mutation in RET consistent with MEN2. The clinician explains that constitutive receptor tyrosine kinase signaling increases MAPK pathway activity and transcription of proliferation genes. How does the identified mutation affect protein function?
Explanation: This question assesses understanding of molecular genetics and gene expression, focusing on proto-oncogene mutations in hereditary cancer syndromes. Gene expression is influenced by receptor tyrosine kinases like RET, which activate MAPK pathways for cell proliferation when mutated. In this vignette, the patient's pheochromocytoma, thyroid nodule, and RET mutation suggest MEN2, with constitutive signaling. The correct answer describes the gain-of-function effect on tyrosine kinase activity, increasing transcription of growth genes. Choice D is incorrect as it pertains to osteogenesis imperfecta, unrelated to the endocrine tumors here. Encourage students to correlate mutation types with functional impacts and screen for associated cancers. Practice integrating biochemical tests with genetic findings for diagnosis.
A 6-year-old boy has congenital heart defects, cleft palate, and recurrent infections with low T-cell counts. He has hypocalcemia. Family history is negative. Karyotype is 46,XY. Chromosomal microarray shows a 22q11.2 deletion involving TBX1. The geneticist explains that TBX1 is a transcription factor important for pharyngeal arch development; reduced TBX1 alters downstream developmental gene expression. Which of the following is most likely disrupted in this patient?
Explanation: This question assesses understanding of molecular genetics and gene expression, focusing on haploinsufficiency in microdeletion syndromes. Gene expression during embryogenesis is controlled by dosage-sensitive transcription factors like TBX1 for pharyngeal development. In this vignette, the boy's heart defects, cleft palate, and 22q11.2 deletion indicate DiGeorge syndrome, reducing TBX1 dosage. The correct answer explains disrupted transcription factor dosage altering developmental genes. Choice E is incorrect as it describes Huntington, unrelated to hypocalcemia. Encourage students to evaluate immune function and use microarray. Practice distinguishing microdeletion phenotypes.
A 35-year-old man is evaluated for recurrent spontaneous pneumothoraces and multiple lung cysts on CT. He has numerous small, dome-shaped papules on his nose and cheeks. His mother had bilateral renal tumors at age 50. The patient’s brother has similar skin lesions. Physical exam is otherwise normal. Karyotype is 46,XY. Targeted molecular analysis reveals a heterozygous pathogenic variant in FLCN consistent with Birt-Hogg-Dubé syndrome. The genetic counselor explains that folliculin acts as a tumor suppressor and influences transcriptional programs by modulating nutrient-sensing pathways, including AMPK–mTOR balance. Which of the following is most likely disrupted in this patient?
Explanation: This question assesses understanding of molecular genetics and gene expression, focusing on nutrient-sensing pathways and their role in syndromic disorders. Gene expression is regulated by environmental cues, with pathways like mTOR balancing cell growth in response to energy availability via tumor suppressors. In this vignette, the patient's pneumothoraces, lung cysts, skin papules, and FLCN variant point to Birt-Hogg-Dubé syndrome, disrupting AMPK-mTOR balance. The correct answer highlights the loss of mTOR restraint under low energy, leading to aberrant growth in lungs and kidneys. Choice A is incorrect as it pertains to Lynch syndrome with microsatellite instability, not relevant to the pulmonary and dermatologic features here. Encourage students to map genetic defects to downstream pathways and consider family history for autosomal dominant patterns. Practice differentiating similar syndromes by key clinical and molecular features.
A 33-year-old woman has numerous colorectal adenomas on screening colonoscopy. She also has osteomas of the jaw and multiple epidermoid cysts. Her father died of colon cancer at age 41. Karyotype is 46,XX. Molecular testing identifies a heterozygous truncating mutation in APC, consistent with familial adenomatous polyposis. The physician explains that APC normally promotes β-catenin degradation; loss leads to nuclear β-catenin and increased transcription of proliferation genes. Which of the following is most likely disrupted in this patient?
Explanation: This question assesses understanding of molecular genetics and gene expression, focusing on Wnt signaling in hereditary colorectal cancer. Gene expression in the intestine is controlled by Wnt pathways, where APC degrades β-catenin to prevent proliferation gene transcription. In this vignette, the patient's adenomas, osteomas, and APC mutation suggest familial adenomatous polyposis, with accumulated β-catenin. The correct answer identifies the disrupted β-catenin degradation, leading to increased Wnt target genes. Choice B is incorrect as it relates to xeroderma pigmentosum, not matching the colonic polyps. Encourage students to recognize extracolonic manifestations and recommend surveillance. Practice interpreting colonoscopy results with genetic data.
A 24-year-old man presents with recurrent kidney stones, bone pain, and fatigue. Labs show hypercalcemia and elevated PTH. MRI reveals a pituitary microadenoma. His father had “parathyroid surgery” in his 30s. Physical exam is unremarkable. Karyotype is 46,XY. Germline sequencing identifies a heterozygous pathogenic variant in MEN1. The physician explains that menin normally regulates transcription by interacting with histone-modifying complexes and restraining cell cycle progression. Loss of menin increases expression of growth-promoting genes in endocrine tissues. What is the mechanism of the genetic defect in this condition?
Explanation: This question assesses understanding of molecular genetics and gene expression, focusing on inheritance mechanisms in tumor suppressor gene disorders. Gene expression is controlled by histone modifications and cell cycle regulators, with tumor suppressors like menin preventing unchecked proliferation in endocrine cells. In this vignette, the patient's hypercalcemia, pituitary adenoma, and MEN1 variant suggest multiple endocrine neoplasia type 1, requiring biallelic inactivation. The correct answer captures the autosomal dominant loss with second-hit, leading to dysregulated growth gene expression. Choice C is incorrect as it describes repeat expansions in conditions like Huntington disease, not fitting the endocrine tumor pattern. Encourage students to apply Knudson's two-hit hypothesis and evaluate family histories for penetrance. Practice distinguishing endocrine syndromes by molecular and clinical profiles.
A 26-year-old woman has recurrent epistaxis and visible telangiectasias on the lips and tongue. She reports exertional dyspnea; CT angiography shows pulmonary arteriovenous malformations. Her mother had similar nosebleeds. Karyotype is 46,XX. Genetic testing reveals a heterozygous pathogenic variant in ENG (endoglin), consistent with hereditary hemorrhagic telangiectasia. The physician explains that endoglin is part of the TGF-β/BMP signaling complex in endothelial cells, influencing SMAD-dependent transcription required for normal vessel development. Which of the following is most likely disrupted in this patient?
Explanation: This question assesses understanding of molecular genetics and gene expression, focusing on TGF-β signaling in vascular disorders. Gene expression in endothelium is regulated by TGF-β/BMP via SMADs, with endoglin facilitating receptor complexes for vessel maturation. In this vignette, the woman's telangiectasias, AVMs, and ENG variant indicate hereditary hemorrhagic telangiectasia, impairing SMAD signaling. The correct answer explains the disrupted endothelial SMAD pathway, leading to abnormal vascular transcription. Choice B is incorrect as it describes pemphigus vulgaris, unrelated to the bleeding diathesis. Encourage students to screen for visceral AVMs and review family histories. Practice integrating imaging with molecular diagnostics.
A 3-year-old girl has normal early development followed by loss of speech and purposeful hand use, with repetitive hand-wringing movements. She develops seizures and gait instability. Family history is negative. Karyotype is 46,XX. Sequencing shows a pathogenic variant in MECP2. The clinician explains that MeCP2 normally represses transcription by binding methylated CpG sites and recruiting histone deacetylases; mutation causes inappropriate gene expression in neurons. How does the identified mutation affect protein function?
Explanation: This question assesses understanding of molecular genetics and gene expression, focusing on epigenetic modifiers in autism-spectrum disorders. Gene expression is modulated by MeCP2, which represses transcription via methylated CpG binding and deacetylase recruitment in neurons. In this vignette, the girl's regression, hand-wringing, and MECP2 variant indicate Rett syndrome, with loss of repression. The correct answer describes loss-of-function reducing binding and repression. Choice B is incorrect as it pertains to MEN2, unrelated to stereotypies. Encourage students to track developmental milestones and discuss therapies. Practice identifying mutation effects on protein function.
A 2-year-old boy has profound speech delay, poor eye contact, and repetitive behaviors. He also has macrocephaly. His father had thyroid cancer in his 30s. Karyotype is 46,XY. Sequencing identifies a heterozygous pathogenic variant in PTEN. The clinician explains that PTEN normally dephosphorylates PIP3, limiting PI3K-AKT signaling and downstream transcriptional programs promoting growth and survival. Which gene expression pathway alteration explains the phenotype?
Explanation: This question assesses understanding of molecular genetics and gene expression, focusing on phosphatase regulation in overgrowth syndromes. Gene expression for growth is inhibited by PTEN dephosphorylating PIP3 to limit PI3K-AKT signaling. In this vignette, the boy's macrocephaly, autism, and PTEN variant suggest PTEN hamartoma syndrome, with increased signaling. The correct answer identifies enhanced PI3K-AKT transcription promoting growth. Choice D is incorrect as it describes Marfan, not matching macrocephaly. Encourage students to screen for cancers and review signaling pathways. Practice correlating behavioral features with genetics.
A 6-year-old child has retinoblastoma diagnosed after leukocoria is noted on a school vision screen. Family history reveals the father lost an eye in infancy due to the same cancer. MRI shows an intraocular mass without metastasis. Karyotype is 46,XX. Germline testing identifies a heterozygous pathogenic variant in RB1. The oncologist explains that pRb normally binds E2F, repressing transcription of S-phase genes; loss allows E2F-driven transcription and uncontrolled cell cycle entry. Which gene expression pathway alteration explains the phenotype?
Explanation: This question assesses understanding of molecular genetics and gene expression, focusing on cell cycle regulators in hereditary cancers. Gene expression for cell cycle progression is repressed by pRb binding E2F, preventing S-phase entry in retinal cells. In this vignette, the child's retinoblastoma and RB1 variant suggest hereditary retinoblastoma, with loss of E2F repression. The correct answer identifies increased transcription of S-phase genes due to absent pRb. Choice D is incorrect as it pertains to BRCA-associated cancers, not matching the ocular tumor. Encourage students to apply two-hit models and discuss enucleation criteria. Practice correlating germline mutations with tumor risks.
A 7-year-old boy has multiple café-au-lait macules, axillary freckling, and several soft cutaneous nodules. He has learning difficulties. His mother has similar skin findings. Ophthalmology notes Lisch nodules. Karyotype is 46,XY. Genetic testing identifies a heterozygous pathogenic variant in NF1. The clinician explains that neurofibromin normally inactivates Ras by enhancing GTP hydrolysis; loss increases MAPK signaling and transcription of growth genes. Which gene expression pathway alteration explains the phenotype?
Explanation: This question assesses understanding of molecular genetics and gene expression, focusing on Ras signaling in neurocutaneous syndromes. Gene expression for cell growth is regulated by Ras GTPases, with neurofibromin enhancing hydrolysis to inactivate Ras-MAPK pathways. In this vignette, the boy's café-au-lait spots, neurofibromas, and NF1 variant indicate neurofibromatosis type 1, with constitutive Ras signaling. The correct answer explains the increased MAPK activity from lost GTPase function, promoting tumor gene expression. Choice B is incorrect as it describes Ehlers-Danlos syndrome, unrelated to the pigmented lesions. Encourage students to use diagnostic criteria and monitor for complications. Practice differentiating neurocutaneous disorders by molecular mechanisms.
A 9-year-old boy has intellectual disability, long face, large ears, and macroorchidism. He has autistic behaviors and poor eye contact. His maternal uncle has similar features. Karyotype is 46,XY. Molecular testing shows hypermethylation of the FMR1 promoter with >200 CGG repeats, consistent with fragile X syndrome. The clinician notes that promoter methylation silences transcription, reducing FMRP and causing dysregulated synaptic protein translation. What is the mechanism of the genetic defect in this condition?
Explanation: This question assesses understanding of molecular genetics and gene expression, focusing on repeat expansions and epigenetic silencing. Gene expression can be silenced by promoter hypermethylation from CGG expansions in FMR1, reducing FMRP for synaptic translation control. In this vignette, the boy's intellectual disability, macroorchidism, and FMR1 repeats indicate fragile X syndrome, with transcriptional silencing. The correct answer describes the CGG expansion causing methylation and reduced transcription. Choice B is incorrect as it pertains to Duchenne dystrophy, not matching the behavioral features. Encourage students to recognize X-linked patterns and test for expansions. Practice integrating physical exam with molecular diagnostics.
A 10-year-old boy is evaluated for progressive dystonia, dysarthria, and developmental regression. MRI shows basal ganglia abnormalities. Family history is negative. Karyotype is 46,XY. Whole-exome sequencing identifies a de novo heterozygous missense mutation in MECP2 affecting the methyl-CpG-binding domain. The neurologist explains that MeCP2 binds methylated DNA and recruits corepressors and histone deacetylases, altering chromatin and transcription in neurons. Which gene expression pathway alteration explains the phenotype?
Explanation: This question assesses understanding of molecular genetics and gene expression, focusing on chromatin regulators in neurodevelopment. Gene expression in neurons is repressed by MeCP2 binding methylated DNA to recruit deacetylases for chromatin compaction. In this vignette, the boy's dystonia, regression, and MECP2 mutation suggest atypical Rett syndrome, impairing repression. The correct answer identifies disrupted methylated-DNA binding, altering neuronal transcription. Choice D is incorrect as it describes FAP, unrelated to basal ganglia issues. Encourage students to recognize de novo mutations and monitor progression. Practice correlating MRI with genetic findings.
A 2-week-old male newborn has poor feeding, hypotonia, and failure to thrive. Physical exam shows hypogonadism and weak cry. Family history is unremarkable. Karyotype is 46,XY. Methylation analysis of 15q11-q13 shows only the maternal methylation pattern, consistent with loss of paternally expressed genes. The geneticist explains that imprinting affects transcription by methylation-dependent silencing of one parental allele. Which gene expression pathway alteration explains the phenotype?
Explanation: This question assesses understanding of molecular genetics and gene expression, focusing on epigenetic regulation in imprinting disorders. Gene expression at 15q11-q13 is controlled by methylation, silencing one allele to allow paternal expression for growth regulation. In this vignette, the newborn's hypotonia, poor feeding, and loss of paternal imprint suggest Prader-Willi syndrome, disrupting appetite genes. The correct answer explains the loss of paternal expression due to imprinting, leading to phenotypic alterations. Choice B is incorrect as it describes FAP, unrelated to the hypogonadism. Encourage students to analyze methylation patterns and review neonatal presentations. Practice correlating epigenetics with clinical outcomes.
A 1-month-old infant has severe hypotonia, feeding difficulties, and respiratory insufficiency. EMG suggests denervation. Family history reveals a prior sibling death in infancy. Karyotype is 46,XX. Molecular testing shows homozygous deletion of SMN1. The neurologist notes that SMN protein is required for snRNP assembly and proper pre-mRNA splicing; loss alters expression of motor neuron survival genes. Which of the following is most likely disrupted in this patient?
Explanation: This question assesses understanding of molecular genetics and gene expression, focusing on RNA processing in neuromuscular disorders. Gene expression requires proper pre-mRNA splicing via snRNPs assembled by SMN protein in motor neurons. In this vignette, the infant's hypotonia, denervation, and SMN1 deletion indicate spinal muscular atrophy, disrupting splicing. The correct answer explains impaired snRNP assembly altering survival genes. Choice B is incorrect as it relates to imprinting, unrelated to EMG findings. Encourage students to use deletion testing and discuss therapies. Practice differentiating motor neuron diseases by molecular defects.
A 16-year-old girl is referred for evaluation of tall stature, long limbs, pectus excavatum, and progressive scoliosis. She has myopia and an echocardiogram shows dilation of the aortic root. Her father had an aortic dissection at age 38. Physical exam shows positive wrist and thumb signs. Karyotype is 46,XX. Sequencing identifies a heterozygous pathogenic missense variant in FBN1 consistent with autosomal dominant Marfan syndrome. The clinician notes that abnormal fibrillin-1 reduces extracellular sequestration of TGF-β, increasing downstream SMAD-mediated transcription of genes that weaken connective tissue. Which gene expression pathway alteration explains the phenotype?
Explanation: This question assesses understanding of molecular genetics and gene expression, focusing on extracellular matrix proteins and cytokine signaling in connective tissue disorders. Gene expression involves regulatory networks where matrix proteins sequester growth factors, influencing pathways like TGF-β for tissue homeostasis. In this vignette, the patient's tall stature, aortic dilation, and FBN1 variant indicate Marfan syndrome, with increased TGF-β signaling due to poor sequestration. The correct answer explains the excess SMAD-mediated transcription weakening connective tissue, matching the skeletal and vascular phenotype. Choice E is incorrect as it describes neurofibromatosis, unrelated to the arachnodactyly and ectopia lentis seen here. Encourage students to link structural protein defects to signaling alterations and review autosomal dominant inheritance. Practice analyzing echocardiogram and genetic results for syndromic diagnoses.
A 4-year-old boy presents with progressive proximal muscle weakness. A muscle biopsy is performed, and genetic analysis reveals a single nucleotide change within an intron of the dystrophin (DMD) gene. This mutation creates an abnormal splice acceptor site. The subsequent production of a truncated, non-functional protein leads to a diagnosis of Duchenne muscular dystrophy.
This patient's mutation most likely leads to a non-functional protein through which of the following mechanisms?
Explanation: The creation of a new, or 'cryptic,' splice site within an intron can cause the spliceosome to incorrectly process the pre-mRNA. This often leads to the inclusion of a portion of the intron in the mature mRNA. This insertion typically causes a frameshift, leading to a premature stop codon and the production of a truncated, non-functional protein.
A 5-year-old girl is evaluated for developmental delay and seizures. She has a happy demeanor with frequent, unprovoked laughter. Physical examination reveals ataxic gait and microcephaly. Genetic testing reveals a microdeletion on the maternal chromosome 15q11-13. A different disorder, Prader-Willi syndrome, results from a deletion in the same region of the paternal chromosome.
The parent-of-origin-specific expression of genes in this chromosomal region is best explained by which of the following?
Explanation: Genomic imprinting is an epigenetic phenomenon where genes are expressed in a parent-of-origin-specific manner. This is achieved through DNA methylation and histone modifications established in the germline. In the 15q11-13 region, certain genes are expressed only from the paternal chromosome and others only from the maternal chromosome. A deletion on the maternal chromosome leads to Angelman syndrome (as in this patient), while a paternal deletion causes Prader-Willi syndrome.
A pharmaceutical company is developing a new chemotherapy agent. The drug is found to reactivate silenced tumor suppressor genes by promoting a more open, transcriptionally active chromatin structure. The drug achieves this by modifying the charge of histone proteins.
The therapeutic effect of this drug is most likely mediated by the inhibition of which of the following enzymes?
Explanation: Histone deacetylation removes acetyl groups from lysine residues on histone tails, increasing their positive charge. This enhances the interaction between histones and negatively charged DNA, leading to condensed chromatin (heterochromatin) and transcriptional repression. Inhibiting histone deacetylases (HDACs) preserves histone acetylation, resulting in a relaxed chromatin state (euchromatin) that allows for gene expression.
A researcher is studying a novel virus that infects human cells. She observes that while the virus successfully transcribes its genes into mRNA within the host nucleus, these viral mRNAs are not efficiently bound by host cell ribosomes in the cytoplasm. Analysis of the viral mRNA reveals that it lacks a specific modification at its 5' terminus that is typically present on eukaryotic mRNAs.
The absence of which of the following structures is the most likely reason for the impaired translation of the viral mRNA?
Explanation: In eukaryotes, the 7-methylguanosine cap at the 5' end of mRNA is crucial for the initiation of translation. It is recognized by the eukaryotic initiation factor eIF4E, which is part of a complex that recruits the small ribosomal subunit to the mRNA. Without this cap, ribosome binding and subsequent translation are severely impaired.