**Core Concept**
Fine-Needle Aspiration Cytology (FNAC) is a diagnostic procedure that involves the sampling of cells from a lesion using a thin needle, typically for histological examination. It is a minimally invasive technique used to evaluate various types of lesions, including cystic lesions, solid masses, and lymph nodes.

**Why the Correct Answer is Right**
FNAC is particularly useful in evaluating cystic lesions, such as thyroid nodules, breast cysts, and ovarian cysts. It helps in distinguishing between benign and malignant cystic lesions by analyzing the cellular morphology and cytological features. In contrast, FNAC is generally not recommended for lesions with fluid-filled cavities, such as abscesses or cysts with thick, fibrotic walls, as the fluid may be contaminated with debris or bacteria, leading to inaccurate results.

**Why Each Wrong Option is Incorrect**
**Option A:** FNAC can be useful in evaluating thyroid nodules, which are a common indication for this procedure.
**Option B:** FNAC is also useful in distinguishing between benign and malignant lymph nodes, especially in the diagnosis of lymphoma.
**Option C:** FNAC is generally not indicated for lesions with fluid-filled cavities, such as abscesses or cysts with thick, fibrotic walls.

**Clinical Pearl / High-Yield Fact**
FNAC is a valuable diagnostic tool, but its accuracy depends on the expertise of the cytopathologist and the quality of the sample. It is essential to ensure that the sample is representative of the lesion and that the cytopathologist has adequate training and experience in interpreting FNAC results.

**Correct Answer: C. Lesions with fluid-filled cavities.**