**Question:** A 14-year-old intellectually disabled child presented to the OPD for the first time with hyper-extensible finger joints and macro-orchidism along with characteristic facial features. MRI of the brain was done. Which of the following sequence repeats is seen in the above condition:

A. (AGGAGG)n
B. (CAG)n
C. (CTG)n
D. (AT)n

**Correct Answer:** (CAG)n

**Core Concept:**

The presented condition in the question is a clinical scenario involving an intellectual disabled child suffering from a genetic disorder characterized by hyper-extensible finger joints, macro-orchidism, and distinct facial features. This condition is associated with a specific sequence repeat expansion in the DNA of the affected individual.

**Why the Correct Answer is Right:**

The correct answer is (CAG)n because it is associated with a group of genetic disorders known as Myotonic dystrophy (DM). This condition is caused by the expansion of CAG trinucleotide repeats within the CTG gene on chromosome 19. The expansion of CAG repeats leads to the production of a faulty protein that disrupts cellular functions and results in the clinical manifestations seen in the child.

**Why Each Wrong Option is Incorrect:**

A. (AGGAGG)n: This option refers to the sequence repeats associated with Fukuyama congenital muscular dystrophy (FCMD), which is not the correct condition for the presented child.

B. (CAG)n: Although (CAG)n is the correct answer, this option is incorrect because it is associated with Huntington's disease, which is not the correct condition for the presented child.

D. (AT)n: This option refers to the sequence repeats associated with X-linked dominant hyperkalemic periodic paralysis, which is not the correct condition for the presented child.

**Why the Correct Answer is Right:**

The correct answer, (CAG)n, is associated with Myotonic dystrophy (DM), a genetic disorder resulting from the expansion of CAG trinucleotide repeats within the CTG gene on chromosome 19. This expansion leads to the production of a faulty protein, which disrupts cellular functions and results in the clinical manifestations observed in the child, including hyper-extensible finger joints, macro-orchidism, and distinct facial features.

**Why Each Wrong Option is Incorrect:**

A. (AGGAGG)n: This option is incorrect because it refers to the sequence repeats associated with Fukuyama congenital muscular dystrophy (FCMD), which is unrelated to the clinical features presented by the child.

B. (CAG)n: This option is incorrect as it is associated with Huntington's disease, a neurodegenerative disorder caused by the expansion of CAG repeats in the HTT gene, not relevant to the child's clinical features.

D. (AT)n: This option is incorrect as it refers to the sequence repeats associated with X-linked dominant hyperkalemic periodic par