A 42-year-old woman presents with a 6-month history of fatigue, weight gain, and cold intolerance. On examination, she has a firm, diffuse goiter. Thyroid function tests show elevated TSH and low free T4. Anti-TPO antibodies are positive. A thyroid biopsy is performed. The histological specimen shows dense lymphoplasmacytic infiltrate with germinal centers, atrophic follicles, and fibrous stroma. The structure marked **B** in the diagram consists of metaplastic follicular epithelial cells with abundant eosinophilic granular cytoplasm. What is the primary composition of the cytoplasm of the cells marked **B**?

Question

Accepted Answer

A. Abundant mitochondria rich in oxidative enzymes. ## Why "Abundant mitochondria rich in oxidative enzymes" is right

The cells marked **B** are Hurthle cells (Askanazy or oncocytic cells), which are metaplastic follicular epithelial cells characteristic of Hashimoto thyroiditis. The hallmark feature of these cells is their abundant eosinophilic granular cytoplasm, which is composed primarily of numerous mitochondria rich in oxidative enzymes and cristae. This gives them their distinctive appearance on light microscopy and explains their oncocytic (mitochondria-rich) nature. This is a pathognomonic histological finding in Hashimoto thyroiditis (Robbins & Cotran Pathologic Basis of Disease 10e — Thyroid section).

## Why each distractor is wrong

- **Rough endoplasmic reticulum for immunoglobulin synthesis**: This would be characteristic of plasma cells and B lymphocytes in the lymphoplasmacytic infiltrate (marked **A**), not the follicular epithelial cells marked **B**.
- **Lysosomes containing peroxidase granules**: While anti-TPO antibodies target thyroid peroxidase, the cytoplasmic granules in Hurthle cells are mitochondria, not lysosomes. Peroxidase is an enzyme in the follicular epithelium, not the defining feature of Hurthle cell cytoplasm.
- **Glycogen and lipid droplets for energy storage**: This would be seen in cells undergoing metabolic stress or degeneration, not in the metaplastic Hurthle cells, which have a specific mitochondrial-rich composition.

**High-Yield:** Hurthle cells = oncocytic cells = mitochondria-rich metaplastic follicular epithelium = pathognomonic for Hashimoto thyroiditis on histology.

[cite:Robbins & Cotran Pathologic Basis of Disease 10e — Thyroid]

Answer

B. Rough endoplasmic reticulum for immunoglobulin synthesis

Answer

C. Lysosomes containing peroxidase granules

Answer

D. Glycogen and lipid droplets for energy storage

Explanation

Why "Abundant mitochondria rich in oxidative enzymes" is right

Why each distractor is wrong

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