**Question:** Least common symptoms in Friedrich's ataxia is

A. Gait instability
B. Dysarthria
C. Loss of sense of smell
D. Sleep apnea

**Core Concept:** Friedrich's ataxia is a dominantly inherited neurodegenerative disorder caused by mutations in the FXN gene, which encodes the protein frataxin. Frataxin is essential for maintaining mitochondrial function and iron homeostasis in the mitochondria.

**Why the Correct Answer is Right:** The least common symptoms in Friedrich's ataxia occur due to the progressive degeneration of spinal cord and cerebellum. The primary symptom is gait instability (ataxia) caused by the dysfunction of the cerebellum. Dysarthria is a common symptom resulting from the involvement of the tongue muscles and speech pathways. Loss of sense of smell (anosmia) is a less common symptom due to involvement of the olfactory bulb and tract. Sleep apnea is a less common symptom and can occur due to the involvement of the respiratory muscles and central respiratory control centers in the brainstem.

**Why Each Wrong Option is Incorrect:**

- Gait instability (ataxia) is a primary symptom of Friedrich's ataxia, making it incorrect as the least common symptom.
- Dysarthria is a common symptom in Friedrich's ataxia due to the involvement of the tongue muscles and speech pathways.
- Anosmia (loss of sense of smell) is less common than ataxia and dysarthria but still occurs due to involvement of the olfactory bulb and tract.
- Sleep apnea is a less common symptom in Friedrich's ataxia, affecting respiratory muscles and central respiratory control centers in the brainstem.

**Clinical Pearl:** Friedrich's ataxia is a progressive neurodegenerative disorder with a wide spectrum of symptoms. While gait instability, dysarthria, and anosmia are common symptoms, sleep apnea is a less common manifestation that can occur due to the involvement of the respiratory muscles and central respiratory control centers in the brainstem. A thorough physical examination should be performed to identify these less common symptoms, allowing for early diagnosis and management of the disease.