**Core Concept**
Illuminated **Frenzel glasses** are used in the diagnosis of **nystagmus** and **vertigo**, specifically to inhibit **visual fixation**. This allows for a more accurate assessment of the patient's vestibular function. The **vestibular-ocular reflex** is a key component in this evaluation.

**Why the Correct Answer is Right**
The correct answer is related to the use of Frenzel glasses in **electronystagmography** (ENG) or **videonystagmography** (VNG) tests, which are diagnostic tools for evaluating **nystagmus** and **balance disorders**. By inhibiting visual fixation, Frenzel glasses help in identifying the presence and characteristics of nystagmus.

**Why Each Wrong Option is Incorrect**
**Option A:** This choice is incorrect because it does not relate to the primary use of Frenzel glasses.
**Option B:** This option is also incorrect as Frenzel glasses are not primarily used for this purpose.
**Option C:** Similarly, this choice does not accurately describe the main application of Frenzel glasses.
**Option D:** This option is incorrect as it is not the correct answer.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that Frenzel glasses are used to diagnose **nystagmus** and **vertigo** by preventing the patient from visually fixing on objects, thus allowing for a more accurate assessment of their vestibular function.

**Correct Answer:** D. Nystagmus