Increasing severity of mental retardation of male members over generations is a result of-
## Core Concept
The question pertains to the phenomenon of increasing severity of mental retardation in male members over generations, which is a characteristic feature of a specific genetic disorder. This condition is associated with an expansion of a trinucleotide repeat on the X chromosome.
## Why the Correct Answer is Right
The correct answer, **C. Fragile X syndrome**, is right because Fragile X syndrome is caused by an expansion of a CGG repeat in the FMR1 gene on the X chromosome. This expansion leads to methylation and subsequent silencing of the gene, resulting in the absence or deficiency of the FMRP protein. The size of the CGG repeat expansion correlates with the severity of the disorder and the age of onset. As the repeat size increases over generations (a phenomenon known as anticipation), the severity of the mental retardation often worsens, and the age of onset decreases. This condition predominantly affects males, who have only one X chromosome, while females, with two X chromosomes, are often carriers unless they are homozygous for the expansion.
## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because it does not specify a condition known for causing mental retardation with the described characteristic of increasing severity over generations.
- **Option B:** This option is incorrect as it does not relate to a condition characterized by the worsening of mental retardation in males over generations due to a trinucleotide repeat expansion.
- **Option D:** This option is incorrect because it does not accurately represent a condition associated with the specific pattern of inheritance and symptomatology described.
## Clinical Pearl / High-Yield Fact
A key point to remember is that **Fragile X syndrome** is the most common cause of inherited intellectual disability and the most common known genetic cause of autism or autism spectrum disorders. The disorder is characterized by physical features such as a long face, large ears, and macroorchidism in post-pubertal males. A critical aspect for diagnosis is the presence of a family history suggestive of X-linked inheritance.
## Correct Answer: C. Fragile X syndrome