Familial dysautonomia is characterised by absence of:
**Core Concept**
Familial dysautonomia, also known as Riley-Day syndrome, is a rare, genetic disorder affecting the development of the autonomic nervous system. This condition leads to various systemic complications due to impaired autonomic function.
**Why the Correct Answer is Right**
The correct answer is related to the absence of a crucial enzyme responsible for the synthesis of neurotransmitters in the autonomic nervous system. Familial dysautonomia is characterized by the absence of the enzyme **tyrosine hydroxylase**, which is essential for the conversion of tyrosine to L-DOPA, a precursor for catecholamine synthesis. This deficiency disrupts the production of neurotransmitters, such as dopamine, norepinephrine, and epinephrine, leading to autonomic dysfunction.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not directly related to the pathophysiology of familial dysautonomia.
* **Option B:** This option is incorrect as it is not the primary enzyme deficiency associated with familial dysautonomia.
* **Option C:** This option is not a correct answer, as the absence of this enzyme is not the hallmark of familial dysautonomia.
**Clinical Pearl / High-Yield Fact**
Familial dysautonomia is a rare condition, but it is essential to recognize its distinctive clinical features, including impaired autonomic function, gastrointestinal dysmotility, and temperature regulation abnormalities.
**Correct Answer:** D.