**Question:** All of the following are histomorphological features of Graves disease, except:

A. Increased cellularity
B. Enlarged follicles
C. Increased secretory activity
D. Thyroid-stimulating immunoglobulins (TSI)

**Core Concept:**
Graves disease is an autoimmune disorder characterized by the production of thyroid-stimulating immunoglobulins (TSI) by the patient's immune system, which directly stimulates the thyroid gland leading to hyperthyroidism. Histomorphological features are the visible changes observed in the thyroid tissue under a microscope.

**Why the Correct Answer is Right:**
D. Thyroid-stimulating immunoglobulins (TSI) are not a histomorphological feature, as they are not directly visible under a microscope. TSI are autoantibodies produced in response to the autoimmune attack on the thyroid gland, but they themselves do not contribute to the histomorphological changes observed in Graves disease.

**Why Each Wrong Option is Incorrect:**
A. Increased cellularity: This feature describes an increase in the number of thyroid cells (chiefly thyrocytes) in the thyroid tissue, which occurs due to the infiltration of immune cells in Graves disease.
B. Enlarged follicles: This feature refers to the dilation of thyroid follicles, which is caused by the hyperthyroidism and increased thyroid hormone production in Graves disease.
C. Increased secretory activity: This feature represents the enhanced production of thyroid hormones (T3 and T4) by the thyrocytes due to the stimulation by TSI, which leads to hyperthyroidism in Graves disease.

**Clinical Pearl:**
Histopathology of Graves disease is essential to differentiate it from other conditions like toxic nodular goiter and Hashimoto's thyroiditis. While toxic nodular goiter is characterized by nodules and multinucleated giant cells, Hashimoto's thyroiditis demonstrates lymphocytic infiltration. A thorough understanding of histopathological features helps in accurate diagnosis and appropriate management of the disease.