**Question:** Reye's syndrome is diagnosed by using a stain for which of the following structures?

A. Liver
B. Brain
C. Kidney
D. Lungs

**Core Concept:** Reye's syndrome is a rare, severe illness that affects children and is characterized by acute encephalopathy (brain dysfunction) and fatty liver degeneration. It is typically associated with aspirin use during viral infections, particularly influenza or gastroenteritis.

**Why the Correct Answer is Right:** The correct answer, B. Brain, is due to the central feature of Reye's syndrome being acute encephalopathy, which is a condition characterized by severe brain dysfunction. Aspirin use in children can lead to Reye's syndrome, and the brain is affected in this condition.

**Why Each Wrong Option is Incorrect:**

A. Liver (Option A) - While the liver can be affected in Reye's syndrome, the primary focus of the question is the brain, not the liver. The liver is affected due to fatty liver degeneration, but the main diagnostic criterion is brain dysfunction.

B. Kidney (Option C) - Although the kidneys can be involved in Reye's syndrome, the question is about the primary structure affected, which is the brain.

D. Lungs (Option D) - While Reye's syndrome can cause respiratory complications, the question is specifically asking about the structure affected in the diagnosis of Reye's syndrome. The lungs are not directly related to the diagnosis of Reye's syndrome.

**Clinical Pearl:** The correct answer, B. Brain, highlights the importance of understanding the specific structures involved in a disease process for accurate diagnosis and management. In the case of Reye's syndrome, knowing the primary structure affected (brain) is crucial for identifying and managing this rare but severe condition in children, particularly those receiving aspirin during viral illnesses.