**Question:** FNAC can diagnose all of the following thyroid lesions except

A. Hashimoto's thyroiditis
B. Multinodular goiter
C. Thyroiditis
D. Thyroid abscess

**Core Concept:** Fine Needle Aspiration Cytology (FNAC) is a diagnostic procedure used in medicine to obtain cells or tissue samples from a body organ, such as the thyroid gland, for microscopic examination. In the context of thyroid lesions, FNAC is a valuable tool for diagnosis and classification of various thyroid disorders.

**Why the Correct Answer is Right:** FNAC is an effective method for diagnosing thyroid lesions, including benign and malignant lesions. However, one cannot diagnose a thyroid abscess using FNAC due to the unique characteristics of an abscess. An abscess is a localized collection of pus within tissues, which is surrounded by a fibrous capsule and separated from the general lymphatic system. In this context, FNAC would not be able to identify the pus or the fibrous capsule characteristic of an abscess.

**Why Each Wrong Option is Incorrect:**

A. Hashimoto's thyroiditis: Hashimoto's thyroiditis is an autoimmune disorder causing chronic inflammation of the thyroid gland. FNAC can identify the characteristic lymphocytic infiltration and atrophy of the thyroid follicles, which are key features of Hashimoto's thyroiditis.

B. Multinodular goiter: Multinodular goiter refers to a thyroid gland with multiple nodules. FNAC can identify the presence of nodules and provide information about their characteristics, such as cellularity, cytology, and pattern, which can help determine whether the nodules are benign or malignant.

C. Thyroiditis: Thyroiditis refers to inflammation of the thyroid gland. Similar to Hashimoto's thyroiditis, FNAC can identify the characteristic lymphocytic infiltration and atrophy of the thyroid follicles, which are key features of thyroiditis.

D. Thyroid abscess: As explained above, thyroid abscess is a localized collection of pus within tissues, surrounded by a fibrous capsule, and separated from the general lymphatic system. FNAC is unable to identify these characteristics due to its inability to visualize the pus and fibrous capsule.

**Clinical Pearl:** Fine needle aspiration cytology is an essential tool for diagnosing thyroid lesions. However, it has certain limitations, such as not being able to diagnose an abscess due to the unique characteristics of the lesion. When evaluating thyroid lesions, a combination of clinical assessment, laboratory tests, and FNAC results, along with appropriate imaging studies (ultrasound, CT, or MRI), can lead to accurate diagnosis and management decisions in thyroid conditions.