**Core Concept**
Pemphigus vulgaris is a chronic autoimmune skin disease characterized by the formation of blisters on the skin and mucous membranes. The Tzank smear is a diagnostic tool used to identify the presence of acantholytic cells, which are a hallmark of pemphigus vulgaris.

**Why the Correct Answer is Right**
The Tzank smear is a cytopathological test that involves examining a scraping of the base of a blister under a microscope. In pemphigus vulgaris, the acantholytic cells show characteristic features such as rounded, keratin-filled cells with a loss of intercellular bridges. This is due to the autoantibodies against desmoglein 3, a protein that plays a crucial role in maintaining the integrity of the skin epithelium.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the Tzank smear is not typically used to diagnose other autoimmune diseases such as lupus or rheumatoid arthritis, which may present with skin manifestations but are not characterized by acantholytic cells.

**Option B:** This option is incorrect because the Tzank smear is not primarily used to diagnose fungal or bacterial infections, which may cause skin lesions but do not result in acantholytic cells.

**Option C:** This option is incorrect because the Tzank smear is not typically used to diagnose skin conditions such as eczema or psoriasis, which may present with skin lesions but do not result in acantholytic cells.

**Clinical Pearl / High-Yield Fact**
The Tzank smear is a rapid and inexpensive diagnostic tool that can be used to differentiate pemphigus vulgaris from other skin conditions. It is essential to perform a Tzank smear in cases of suspected pemphigus vulgaris to confirm the diagnosis and guide treatment.

**Correct Answer:** C. Acantholytic cells with rounded, keratin-filled morphology.