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0%
3%
5/197
25%
35%
68/197
50%
49%
96/197
75%
1%
1/197
100%
4%
7/197
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This patient with hemarthrosis and a history of bruising with an isolated increase in partial thromboplastin time most likely has hemophilia. This disease is X-linked recessive so his younger brother would have a 50% chance of inheriting the mutated allele from his mother. Hemophilia is caused by an X-linked recessive deficiency in coagulation factors that leads to a defect in secondary hemostasis. Boys will, therefore, have a 50% chance of inheriting a defective allele from a carrier mother while girls will not be affected unless both parents pass a defective allele. Problems with secondary hemostasis result in deep bleeding such as hematomas, hemarthrosis, bruises, and prolonged bleeding after surgery. Laboratory studies will reveal a normal bleeding time because platelets are not affected in this disorder, and a normal prothrombin time because the extrinsic coagulation pathway is also unaffected. This will result in an isolated increase in partial thromboplastin time. The most common version is hemophilia A, which is caused by a deficiency of factor VIII and can be treated with factor VIII replacement. Incorrect Answers: Answer 1: 0% would be correct for a female sibling because an unaffected father must have a normal X chromosome and would, therefore, pass along a normal allele. Answer 2: 25% would be correct for an autosomal recessive disease because both carrier parents would have to pass a defective allele for the child to inherit the disease. The chance for each parent to pass the allele is 50% so the combined probability is 50% * 50% = 25%. Hemophilia is an X-linked recessive disease. Answer 4: 75% would be correct for an autosomal dominant disease from 2 affected parents because inheriting either defective allele would lead to disease development. Hemophilia is an X-linked recessive disease. Answer 5: 100% would be correct if the mother was affected because the inheritance of either allele would lead to disease development. Since this mother is not affected, she must instead be a carrier for the disease. Bullet Summary: Hemophilia most commonly affects boys because it is inherited in an X-linked recessive manner.
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