Free Online USMLE Step 1 Questions and Answers 5

10. A 50-year-old previously healthy man is evaluated for progressive fatigue, weakness, and recurrent gingival bleeding. Laboratory studies reveal normocytic normochromic anemia, thrombocytopenia, and leukocytosis with circulating myeloblasts. Bone marrow biopsy establishes a diagnosis of acute myeloid leukemia. Induction chemotherapy followed by allogeneic hematopoietic cell transplantation (HCT) is planned. Molecular typing of human leukocyte antigen (HLA) -A, -B, -C, -DP, -DQ, and -DR is performed. The patient’s biological sister, with whom he shares both parents, is eligible for stem cell donation and undergoes HLA typing. Which of the following is the most likely probability that the sibling will be an identical HLA match with this patient?

A. 0
B. 1/16
C. 1/8
D. 1/4
E. 1/2

Correct Answer: D

Answer Explanation:

Although there are thousands of HLA alleles in the human population with millions of potential combinations, the HLA genes are clustered within a short region of a single chromosome. This results in a low rate of crossover, allowing the HLA gene cluster to be treated as an HLA haplotype (a series of linked genes on the same chromosome). Each child inherits 2 HLA haplotypes, one from the mother and one from the father. Therefore, the probabilities that a given sibling wi ll share some or all of the same HLA genes are as follows:
• 1/4 chance of inheriting all the same HLA genes (ie, identical HLA match).
• 1 /2 chance of inheriting half of the same HLA genes (ie, haploidentical HLA match) (Choice E).
• 1/4 chance of inheriting none of the same HLA genes (ie, HLA mismatch).

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