**Core Concept:** Inheritance in Mendelian Genetics

Inheritance refers to the transmission of genetic traits from parents to offspring. In the context of multiple-choice question (MCQ), the possible modes of inheritance are autosomal dominant, autosomal recessive, X-linked dominant, X-linked recessive, and sex-linked recessive. This question focuses on the inheritance pattern of a rare disease called kinky hair disease (also known as "Hairless-like 1 homozygous mouse" or "Akita syndrome").

**Why the Correct Answer is Right:**

Kinky hair disease is an autosomal recessive disorder, which means that an affected individual must inherit two copies of the defective gene (one from each parent) to develop the disease. This mode of inheritance is consistent with the pattern observed in Mrs. A's family history.

**Why Each Wrong Option is Incorrect:**

A. Autosomal dominant inheritance (Option A) would require only one defective gene to manifest the condition, which is inconsistent with Mrs. A's family history.

B. X-linked dominant inheritance (Option B) would involve a male offspring being affected, which is not consistent with the family's female offspring being affected.

C. X-linked recessive inheritance (Option C) would require both parents to be carriers, and the affected offspring would be a male, which is not consistent with the family history.

D. Sex-linked recessive inheritance (Option D) involves inheritance of the gene from a parent to an offspring, which is not consistent with the pattern observed in Mrs. A's family history.

**Clinical Pearl:**

Understanding the inheritance pattern of genetic disorders is essential for genetic counselling and preventing further cases in future generations. If a couple planning a family knows the mode of inheritance and the affected family members, they can take appropriate measures to reduce the risk of having a child with the disease. This might include preimplantation genetic diagnosis (PGD), selective abortions, or choosing a healthy partner with no history of the disease.

**Correct Answer:**

D. Sex-linked recessive inheritance

Kinky hair disease is a rare genetic disorder caused by mutations in the KRT71 gene, which encodes keratin 1, a protein essential for hair structure. The disease is inherited in an X-linked recessive pattern, meaning that the disease occurs when a female inherits two copies of the defective gene (one from each parent) and a male receives the gene from an affected mother (heterozygous carrier) and a normal father. This inheritance pattern explains the family history provided in the MCQ.