## Core Concept
Idiopathic sudden sensorineural hearing loss (ISSNHL) is a medical emergency characterized by a rapid decline in hearing, typically affecting one ear. The condition involves a sudden loss of inner ear function, leading to sensorineural hearing loss. The exact cause remains unknown, but various theories suggest vascular compromise, viral infections, or immune-mediated mechanisms.

## Why the Correct Answer is Right
The correct answer involves understanding the clinical presentation and management of ISSNHL.
- **Vertigo is not always present**: While vertigo can occur, it is not a universal symptom in ISSNHL.
- **Carbogen (5% CO2 + 95% O2) is beneficial**: Some studies suggest that carbogen inhalation may improve oxygenation of the inner ear and enhance recovery in ISSNHL.
- **Hearing loss occurs within 72 hours**: The hearing loss in ISSNHL typically occurs over a short period, often defined as within 72 hours.
- **Audiometric threshold changes occur over at least 3 frequencies**: A diagnosis of ISSNHL requires a significant hearing threshold shift in at least three consecutive frequencies on audiometry.

## Why Each Wrong Option is Incorrect
- **Option A: Vertigo always present** - This is incorrect because while some patients with ISSNHL may experience vertigo, it is not a universal symptom.
- **Option C: Hearing loss occur within 24 hours** - Although hearing loss can occur within 24 hours, the more accepted timeframe for ISSNHL diagnosis is within 72 hours.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that **idiopathic sudden sensorineural hearing loss is a medical emergency**, and prompt treatment, often with corticosteroids, is crucial for optimal recovery. Early intervention may significantly improve outcomes.

## Correct Answer: B.