**Core Concept:**
Otospongiosis is a pathological condition characterized by the formation of spongy, hyperplastic changes in the inner ear's stria vascularis, specifically in the Reissner's membrane and the stria vascularis. This condition is associated with a decrease in the production of endolymph, leading to impaired hearing and balance functions.

**Why the Correct Answer is Right:**
The correct answer, D (hypertrophy of Reissner's membrane), describes the pathological change that occurs in Otospongiosis. In this condition, Reissner's membrane, part of the inner ear, undergoes hyperplasia (i.e., excessive growth or thickening) as a compensatory response to the reduced endolymph production. This results in the formation of spongy, hyperplastic changes in the stria vascularis and Reissner's membrane. This ultimately leads to impaired hearing and balance functions.

**Why Each Wrong Option is Incorrect:**
A) Otospongiosis is not caused by the enlargement of the spiral ligament, which is option A. The spiral ligament is a fibrous structure that connects the modiolus to the oval window and does not contribute to endolymph production or hearing impairment.

B) Otospongiosis is not primarily characterized by an increase in the size of the cochlea, which is option B. The condition primarily affects the inner ear structures involved in endolymph production and maintenance of hearing function.

C) Otospongiosis does not result from the enlargement of the stria vascularis and Reissner's membrane, which is option C. Otospongiosis is characterized by the hyperplasia of these structures to compensate for reduced endolymph production.

**Clinical Pearl:**
Hypertrophy of Reissner's membrane and stria vascularis in Otospongiosis is a crucial finding in understanding the pathophysiology of the condition. It highlights the body's compensatory response to maintain endolymph production and protect hearing and balance functions despite reduced endolymph production. Recognizing this compensatory response is essential for diagnosing and managing patients with suspected otospongiotic changes on audiological evaluations.