## **Core Concept**
Pyrexia of unknown origin (PUO) refers to a prolonged fever of more than 3 weeks with no obvious cause after an extensive diagnostic workup. The evaluation of PUO involves a systematic approach to identify infectious, inflammatory, neoplastic, and other causes. A thorough history, physical examination, and laboratory tests are essential in diagnosing PUO.

## **Why the Correct Answer is Right**
The correct answer, , indicates that the statement regarding PUO is false. Typically, PUO is characterized by a temperature elevation of more than 38.3°C (100.9°F) on several occasions, with a duration of more than 3 weeks. The evaluation includes a comprehensive history, physical examination, and initial laboratory tests such as complete blood count (CBC), blood cultures, erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), and urinalysis.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because it likely presents a true statement regarding PUO, which could involve infections, autoimmune diseases, or malignancies as potential causes.
- **Option B:** This option is incorrect as it probably aligns with the definition or common causes of PUO, making it a true statement.
- **Option C:** This option is incorrect because it might represent another true aspect of PUO, such as the necessity for a detailed diagnostic workup to rule out various causes.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the classic definition of PUO includes a fever of at least 38.3°C (100.9°F) on several occasions, with a duration of more than 3 weeks, and no diagnosis after an outpatient evaluation or 3 days in the hospital. This definition helps guide the diagnostic approach and distinguishes PUO from other types of fever.

## **Correct Answer:** D.