WBR0087

Author	PageAuthor::Ogheneochuko Ajari MB.BS, MS
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Biochemistry, MainCategory::Genetics
Sub Category	SubCategory::Neurology
Prompt	[[Prompt::A 56-year old man presents to the physicians office with a history of difficulty with swallowing, involuntary jerky movements of the hands, progressive loss of speech and memory. His father died of similar symptoms and his symptoms is said to be worse than his father’s. This condition is an example of which of the following?]]
Answer A	AnswerA::Loss of heterozygosity
Answer A Explanation	AnswerAExp::An example loss of heterozygosity is retinoblastoma. If there is an inheritance or development of a mutation in a tumor suppressor gene, the complementary allele must be deleted/mutated before cancer develops.
Answer B	AnswerB::Locus heterogeneity
Answer B Explanation	AnswerBExp::Locus heterogeneity is exhibited by Marfan’s syndrome, MEN 2B and homocystinuria. Mutations at different loci produce the same phenotype.
Answer C	AnswerC::Genomic imprinting
Answer C Explanation	AnswerCExp::Genomic imprinting is seen in Prader-Willi syndrome and Angelman syndrome where the differences in phenotype depend on the origin of the mutation whether it is paternal or maternal.
Answer D	AnswerD::Anticipation
Answer D Explanation	AnswerDExp::Anticipation is exhibited by Huntington disease, where the severity of the disease worsens, or age of onset of disease is earlier in succeeding generations
Answer E	AnswerE::Pleitropy
Answer E Explanation	AnswerEExp::Pleitropy is seen in phenylketonuria where one gene has greater than one effect on an individual’s phenotype.
Right Answer	RightAnswer::D
Explanation	[[Explanation::The patient in this vignette has Huntington’s disease. It is an example of anticipation where the severity of the disease worsens or age of onset of disease is earlier in succeeding generations. Symptoms manifest in affected individuals between the ages of 20 and 50. It is a trinucleotide repeat disorder (CAG)n with the gene located on chromosome 4. Findings include depression, progressive dementia, choreiform movements, caudate atrophy and decreased levels of GABA and Ach in the brain. Educational Objective: References: ]]
Approved	Approved::Yes
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