**Core Concept**
The external auditory canal (EAC) is a complex structure with both bony and cartilaginous components. The cartilaginous part is more susceptible to infections and injuries due to its softer and more flexible nature.

**Why the Correct Answer is Right**
The widening of the cartilaginous part of the EAC is commonly seen in otosclerosis, a type of bone growth in the middle ear that can cause hearing loss. However, when considering other options, we can rule out conditions that primarily affect the bony part of the EAC, such as osteomas or exostoses. **Option C**, a perforated tympanic membrane, is unrelated to the cartilaginous EAC. **Option D**, an impacted cerumen (wax) bolus, may cause a temporary blockage but does not lead to structural changes in the EAC.

**Why Each Wrong Option is Incorrect**
**Option A:** Osteopetrosis is a rare genetic disorder characterized by increased bone density, which would not typically cause widening of the cartilaginous EAC.

**Clinical Pearl / High-Yield Fact**
It's essential to differentiate between the bony and cartilaginous parts of the EAC when evaluating patients with ear symptoms, as the management and prognosis can vary significantly depending on the affected area.

**Correct Answer:** A. Otosclerosis