**Core Concept**
Homocystinuria is a genetic disorder caused by a deficiency in cystathionine beta-synthase (CBS), which is an enzyme involved in the metabolism of homocysteine. This condition is inherited in an autosomal recessive pattern, meaning that a person must inherit two defective copies of the gene (one from each parent) to express the condition.

**Why the Correct Answer is Right**
In autosomal recessive inheritance, each parent carries one normal and one defective copy of the gene. The chance of having a child with Homocystinuria when only one parent is affected is calculated as follows: The affected parent has a 50% chance of passing the defective gene to each child. Since the other parent is normal, they can only contribute a normal gene. Therefore, each child has a 50% chance of inheriting the defective gene from the affected parent.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it implies that the chance of having Homocystinuria is 0% when only one parent is affected. However, since the affected parent can pass the defective gene to each child, there is a 50% chance of inheritance.
**Option B:** This option is incorrect because it suggests that the chance of having Homocystinuria is 25% when only one parent is affected. However, in autosomal recessive inheritance, each child has a 50% chance of inheriting the defective gene, not a 25% chance.
**Option C:** This option is incorrect because it implies that the chance of having Homocystinuria is 100% when only one parent is affected. However, since the other parent is normal, they can only contribute a normal gene, making the chance of inheritance 50%, not 100%.
**Option D:** This option is incorrect because it suggests that the chance of having Homocystinuria is 75% when only one parent is affected. However, in autosomal recessive inheritance, each child has a 50% chance of inheriting the defective gene, not a 75% chance.

**Clinical Pearl / High-Yield Fact**
In autosomal recessive disorders, each child of an affected parent has a 50% chance of inheriting the defective gene. This is in contrast to autosomal dominant disorders, where each child of an affected parent has a 50% chance of expressing the condition.

**Correct Answer:** . 50%