**Core Concept**
Sickle cell anemia is an autosomal recessive genetic disorder caused by a mutation in the HBB gene, leading to the production of abnormal hemoglobin (HbS). This mutation affects the hemoglobin's ability to bind oxygen, resulting in red blood cells that are prone to sickling and destruction.

**Why the Correct Answer is Right**
When both parents are sickle cell anemia patients, they are homozygous recessive for the HBB gene mutation, denoted as HbS/HbS. Each parent has a 100% chance of passing the mutated gene to their offspring. Since both parents are carriers of the same recessive allele, all their offspring will inherit the mutated gene from both parents, making them homozygous recessive (HbS/HbS) and, therefore, affected by the disease.

**Why Each Wrong Option is Incorrect**
**Option A:** 10% is incorrect because it fails to account for the fact that both parents are carriers of the same recessive allele, making it impossible for their offspring to be unaffected.

**Option B:** 25% is incorrect because it assumes a random assortment of alleles during meiosis, which is not the case when both parents are homozygous recessive for the same allele.

**Option C:** 50% is incorrect because it implies that the offspring have a 50% chance of inheriting the normal allele from one parent, which is not possible in this scenario.

**Clinical Pearl / High-Yield Fact**
In autosomal recessive disorders, the likelihood of offspring inheriting the disease is directly related to the genotype of the parents. When both parents are carriers of the same recessive allele, their offspring have a 100% chance of inheriting the disease.

✓ Correct Answer: D. 100%