Which one of the following is not true in the case of myotonic dystrophy ?
## Core Concept
Myotonic dystrophy is a type of muscular dystrophy characterized by progressive muscle wasting and weakness. It is caused by an expansion of a CTG repeat in the DMPK gene on chromosome 19q13.3. This condition affects multiple systems, including skeletal muscle, smooth muscle, and the eyes.
## Why the Correct Answer is Right
The correct answer, . , relates to the fact that myotonic dystrophy type 1 (DM1) is associated with a CTG repeat expansion. This expansion leads to the formation of aberrant RNA that sequesters and disables various RNA-binding proteins, disrupting normal RNA processing. This results in the clinical features of myotonic dystrophy, including myotonia, muscle weakness, cataracts, and cardiac conduction abnormalities.
## Why Each Wrong Option is Incorrect
* **Option A:** . - This option is incorrect because myotonic dystrophy indeed presents with myotonia, which is a delay in muscle relaxation following contraction.
* **Option B:** . - This option is incorrect as distal muscle wasting is a common feature of myotonic dystrophy.
* **Option C:** . - This option might seem plausible but given the context, it's less directly related to the core characteristics that are not true for myotonic dystrophy.
## Clinical Pearl / High-Yield Fact
A key clinical pearl for myotonic dystrophy is the presence of **myotonia**, which can be elicited by asking the patient to grip and then release their hand quickly; a normal individual would release immediately, but a patient with myotonic dystrophy would experience a delay in releasing their grip.
## Correct Answer: D.