**Core Concept:** Autosomal recessive disorders are a group of genetic conditions that are inherited when an individual has two copies of a mutated gene, one from each parent. These conditions occur when both parents are carriers of the mutated gene and pass on one mutated gene to their offspring. Autosomal recessive disorders involve genes located on autosomes, which are non-sex chromosomes.

**Why the Correct Answer is Right:** Option D (Down syndrome) is an example of an autosomal recessive disorder. Down syndrome is caused by the presence of an extra copy of chromosome 21, which leads to an overexpression of specific genes on that chromosome. This results in the characteristic features of Down syndrome, including developmental delays, intellectual disability, and distinct facial features.

**Why Each Wrong Option is Incorrect:**

A. (Turner syndrome, also known as 45,X karyotype, is an example of a chromosomal disorder - it is not autosomal recessive. Turner syndrome is caused by a complete or partial absence of one of the two X chromosomes in females.)

B. (Krabbe disease is caused by mutations in the GALC gene and is an example of an autosomal recessive disorder. Option B is incorrect as it is not the correct answer.)

C. (Cystic fibrosis is caused by mutations in the CFTR gene and is an example of an autosomal recessive disorder. Option C is incorrect as it is not the correct answer.)

**Clinical Pearl:** Understanding the inheritance patterns of genetic disorders is crucial in genetic counseling and predicting the chances of an individual or offspring developing a specific condition. Autosomal recessive disorders can be passed down through families, and individuals can be carriers without showing symptoms. Genetic counselors can help individuals and families understand their risk of having an affected child and provide guidance on prenatal diagnosis, preimplantation genetic diagnosis, or selective termination of pregnancy if needed.