**Question:** The cataract seen in patients with CTG/CUG trinucleotide repeats is:

A. Myotonic dystrophy
B. Huntington's disease
C. Fragile X syndrome
D. Spinocerebellar ataxia

**Core Concept:**

Trinucleotide repeats are repetitions of three nucleotides in the DNA sequence that can expand or contract during cell division, leading to genetic disorders. These expansions are particularly prone to causing diseases due to the instability of the repeats. In some cases, these expansions lead to the formation of toxic structures, which can impair cellular function and contribute to disease manifestations.

**Why the Correct Answer is Right:**

The correct answer is D. Spinocerebellar ataxia (SCA) because it is a group of genetic disorders characterized by degeneration of the cerebellum, spinal cord, and brain stem. CTG/CUG trinucleotide repeats can expand in the SCA genes, leading to the formation of toxic RNA foci that sequester RNA binding proteins, disrupting cellular function and causing the characteristic cerebellar ataxia (involuntary tremors and uncoordinated movements) and extrapyramidal symptoms (movement disorders).

**Why Each Wrong Option is Incorrect:**

A. Myotonic dystrophy (MD): MD is a genetic disorder caused by CTG trinucleotide repeats in the DM1 gene, leading to the production of a toxic RNA molecule that sequesters proteins, causing muscle weakness, myotonia, and other symptoms. MD is not associated with cataracts as seen in the correct answer.

B. Huntington's disease (HD): HD is caused by an expanded CAG trinucleotide repeat in the HTT gene, leading to the production of an abnormal protein that causes neuronal death, predominantly in the striatum and cortex. HD is also not associated with cataracts.

C. Fragile X syndrome: Fragile X syndrome is caused by an expanded CGG trinucleotide repeat in the FMR1 gene, leading to a deficiency of the FMRP protein, causing intellectual disability, macroorchidism, and distinctive facial features. Fragile X syndrome is not associated with cataracts.

**Clinical Pearl:**

Cataracts are a known complication in patients with certain genetic disorders, such as SCA1, SCA3, and SCA6, which are caused by CTG repeats in the ATXN1, ATXN3, and CACNA1S genes, respectively. These genetic mutations lead to the production of toxic RNA or proteins causing cellular dysfunction and death, leading to neurological symptoms and possible ocular manifestations like cataracts.