**Core Concept**
Otosclerosis is a type of bone growth in the middle ear that can cause hearing loss. It primarily affects the stapes bone, leading to fixation and subsequent conduction of sound. This condition is characterized by a progressive sensorineural hearing loss.

**Why the Correct Answer is Right**
Rinne's test is used to assess the difference in bone conduction and air conduction of sound. In otosclerosis, bone conduction is better than air conduction due to the fixation of the stapes bone, resulting in a negative Rinne's test. A positive Rinne's test indicates that air conduction is better than bone conduction, which is not characteristic of otosclerosis. This is because the normal conductive pathway is disrupted in otosclerosis, leading to impaired air conduction.

**Why Each Wrong Option is Incorrect**
**Option A:** Conductive deafness is a characteristic feature of otosclerosis due to the fixation of the stapes bone, which disrupts the normal conduction of sound.
**Option C:** Paracusis willisii is a phenomenon observed in patients with otosclerosis, where they can hear better in noisy environments. This is due to the compensatory mechanism of the brain, which tries to amplify sound in an attempt to overcome the conductive hearing loss.
**Option D:** A mobile eardrum is not a characteristic feature of otosclerosis, as the disease primarily affects the stapes bone and the middle ear ossicles.

**Clinical Pearl / High-Yield Fact**
Otosclerosis is the most common cause of conductive hearing loss in young adults. It often presents with a gradual decline in hearing, particularly in the high-frequency range. A thorough otoscopic examination and audiometric testing are essential for diagnosing otosclerosis.

**✓ Correct Answer: B. Positive Rinne's test**