## Core Concept
Haemophilia A, also known as classic haemophilia, is a genetic disorder caused by missing or defective **factor VIII**, a clotting protein. This condition leads to prolonged bleeding, particularly into joints and muscles. It is an X-linked recessive disorder, primarily affecting males.

## Why the Correct Answer is Right
The correct answer, ., relates to the fact that Haemophilia A is caused by a deficiency in factor VIII. This condition results from mutations in the F8 gene, which encodes for factor VIII. The deficiency impairs the blood clotting cascade, leading to the characteristic bleeding manifestations.

## Why Each Wrong Option is Incorrect
- **Option A:** If option A states something like "Haemophilia A is caused by deficiency of factor VIII," then it is correct and not the answer we're looking for because it accurately describes Haemophilia A.
- **Option B:** Without the specific details of option B, if it aligns with known facts about Haemophilia A, such as its X-linked inheritance pattern or its prevalence, then it would be incorrect to label it as the "except" option.
- **Option C:** Similarly, if option C provides accurate information regarding Haemophilia A, such as treatment options (e.g., factor VIII replacement therapy) or common complications (e.g., joint disease), it would not be the correct "except" option.
- **Option D:** If option D provides incorrect information about Haemophilia A, such as suggesting it is an autosomal dominant disorder or caused by a deficiency in another clotting factor, then it would be the correct "except" option.

## Clinical Pearl / High-Yield Fact
A key point to remember is that Haemophilia A can be treated with **factor VIII replacement therapy**, which can be prophylactic to prevent bleeding episodes or on-demand to treat active bleeding. Another important aspect is the risk of **inhibitor formation** against factor VIII, which complicates treatment in some patients.

## Correct Answer: D. Desmopressin is used in treatment of haemophilia B.