## Core Concept
Osteoid osteoma is a type of benign bone tumor that is typically small, usually less than 2 cm in size. It is characterized by a central nidus surrounded by a zone of reactive bone sclerosis. The nidus is the area of neoplastic osteoid tissue.

## Why the Correct Answer is Right
The correct answer, , states that osteoid osteoma is typically painful and often causes pain that is worse at night and relieved by NSAIDs. This is a hallmark characteristic of osteoid osteoma. The pain associated with osteoid osteoma can be severe and is often described as a dull ache that worsens at night. This nocturnal pain can be a significant clue to the diagnosis.

## Why Each Wrong Option is Incorrect
* **Option A:** Osteoid osteomas are typically small, usually less than 2 cm in size, making this statement true.
* **Option B:** The nidus of an osteoid osteoma can be lytic, sclerotic, or mixed on radiographs, but it often appears as a well-defined lytic lesion with a surrounding area of sclerosis, making this statement somewhat true but context-dependent.
* **Option C:** This option might state something like "osteoid osteoma is a type of malignant bone tumor," which would be incorrect because osteoid osteoma is benign.
* **Option D:** If this option suggests that osteoid osteomas do not respond to NSAIDs, it would be incorrect because a key diagnostic feature and treatment clue is that the pain of osteoid osteoma often responds well to NSAIDs.

## Clinical Pearl / High-Yield Fact
A memorable point for exams is that osteoid osteomas often cause nocturnal pain that is relieved by nonsteroidal anti-inflammatory drugs (NSAIDs). This characteristic can help distinguish osteoid osteoma from other bone lesions.

## Correct Answer: C.