The absence of which of the following clotting factors leads to haemophilia A?
A. Factor VII
B. Factor VIII
C. Factor IX
D. Factor X

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Hint: Haemophilia is typically an inherited bleeding disorder during which the blood doesn't clot properly. This will cause spontaneous bleeding in case of any injuries or surgery. Blood contains many proteins called clotting factors which will help to prevent bleeding.

Complete answer: Haemophilia A, also called factor (FVIII) deficiency or classic hemophilia, is a genetic disorder caused by the presence of defective factor VIII, a clotting protein. Although it is passed down from parents to children, about thirty-three percent of cases are caused by a spontaneous mutation, a change in a gene.

Hemophilia is mainly the result of a certain mutational event in the gene (located on the X chromosome) that provides instructions for making the clotting factor protein needed to form a blood clot.
Males possess one X and one Y chromosome as a result of which they only have one copy of the genes present on the X chromosome.
Females on the other hand have two copies of the genes on the X chromosome. This implies that males can only have a disease like hemophilia if they inherit an affected X chromosome that has a mutation in either the factor VIII or factor IX gene (causes haemophilia B).
It is possible that females can also have haemophilia, but this condition is rare as in most such cases due to excessive bleeding the fetus dies. Most haemophilic females are carriers of the disease.

So, the correct answer is option B. factor VIII.

Note: In very rare cases, a person may develop hemophilia much later in their life. Most cases of haemophilia are reported in middle-aged or elderly people. It is also common in young women who have recently given birth or those who are in their last phase of pregnancy.