USMLE-Rx Step 1 Practice Q's

USMLE-Rx Step 1 Qmax Challenge #1219

Check out today’s Step 1 Qmax Question Challenge.

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usmle-rx-step-1-qmax-challenge-1219A 2-year-old child has required frequent transfusions throughout his life because of anemia. A peripheral blood smear demonstrates microcytic, hypochromic red cells, with target cells and anisopoikilocytosis. After further genetic testing, the child’s spleen is removed to lessen the need for transfusions. When the suspected causal RNA is amplified and sequenced, the mutation in the image is seen at the 5′ end of one of his introns.

This change is most likely to affect which of the following processes?

A. Capping
B. Hybridization
C. Polyadenylation
D. Splicing
E. Translation

———————–

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3 replies »

  1. The correct answer is D. Introns are noncoding regions of RNA that are spliced out of mature mRNA. Almost all introns begin and end with 5′-GU——AG-3′. A mutation in one of those nucleotides affects splicing. As the mutant shown in the diagram changes from the wild 5′-GU to a 5′-AU, the intron is prevented from being properly spliced at the appropriate site. This type of mutation is one of those found in the thalassemia vignette, as described in this patient.

    A is not correct. Capping of the mRNA occurs at the 5′ end as it is being transcribed from DNA into RNA. A mutation at the 5′ end of an intron would not affect capping.

    B is not correct. Hybridization is a process in which single-stranded DNA base-pairs with a complementary sequence. A mutation at the 5′ end of an intron would not affect hybridization.

    C is not correct. A poly(A) tail is added to the 3′ end of heterogeneous nuclear RNA (hnRNA, the initial RNA transcript) after transcription. Poly(A) polymerase uses ATP as a precursor for adding adenosine one molecule at a time. A mutation at the 5′ end of an intron would not affect polyadenylation.

    E is not correct. Introns are non-protein-coding sections of an RNA transcript. Translation of RNA into protein would not be affected by mutation in an intron.

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