Which of the following is true about fever?

Explanation

## Correct Answer: C. IL-1 is an endogenous pyrogen Interleukin-1 (IL-1) is a canonical **endogenous pyrogen** — a cytokine produced by activated macrophages, monocytes, and dendritic cells during infection or inflammation that acts on the hypothalamic thermoregulatory centre to raise the set-point temperature. IL-1 does not directly raise body temperature; instead, it stimulates hypothalamic neurons to synthesize and release **PGE2** (not PGD2), which resets the anterior hypothalamic temperature set-point upward. This is why fever is a regulated response, not uncontrolled heat generation. Other endogenous pyrogens include TNF-α, IL-6, and IFN-γ. In Indian clinical practice, understanding endogenous pyrogens is critical for interpreting fever in sepsis, tuberculosis, and other chronic infections where IL-1 drives the systemic inflammatory response. The distinction between exogenous pyrogens (bacterial lipopolysaccharides, toxins) and endogenous pyrogens (host cytokines) is fundamental to pathophysiology and explains why antipyretics work downstream of IL-1 action. ## Why the other options are wrong **A. Aspirin increases fever by inhibiting prostaglandin synthesis** — This is backwards. Aspirin and NSAIDs **reduce** fever by inhibiting cyclooxygenase (COX), thereby blocking PGE2 synthesis in the hypothalamus. PGE2 is the final mediator that raises the temperature set-point; blocking its production lowers fever. This is a classic NBE trap that reverses the mechanism to test mechanistic understanding. **B. PGD2 is responsible for resetting the temperature to a higher level** — PGD2 is not the thermoregulatory prostaglandin — **PGE2** is. PGE2, not PGD2, acts on EP3 receptors in the anterior hypothalamus to raise the temperature set-point. PGD2 is involved in sleep-wake regulation and immune suppression. This option confuses prostaglandin subtypes to trap students who know prostaglandins are involved but lack specificity. **D. Fever occurs during inflammation due to release of histamine** — Histamine is a mediator of acute inflammation (vasodilation, increased vascular permeability) but is **not** a pyrogenic mediator. Fever in inflammation is driven by IL-1, TNF-α, and IL-6, not histamine. This option conflates acute inflammatory mediators with pyrogenic cytokines — a common misconception in Indian medical education where histamine is heavily emphasized in allergy chapters. ## High-Yield Facts - **IL-1, TNF-α, IL-6, and IFN-γ** are the four main endogenous pyrogens; IL-1 is the prototype. - **PGE2** (not PGD2) is the final hypothalamic mediator that resets the temperature set-point upward via EP3 receptors. - **Aspirin and NSAIDs** reduce fever by blocking COX → ↓ PGE2 synthesis in the hypothalamus. - **Endogenous pyrogens** are host-derived cytokines; **exogenous pyrogens** are bacterial/microbial products (LPS, peptidoglycans). - Fever is a **regulated hyperthermia** (set-point raised), not uncontrolled heat production — this explains why patients with fever still shiver when cold. ## Mnemonics **Endogenous Pyrogens: TILI** **T**NF-α, **I**L-1, **L**ymphokines (IL-6), **I**FN-γ — the four main host cytokines that trigger fever. IL-1 is the most studied and the prototype endogenous pyrogen. **Fever Pathway: IL → PGE → ↑ Set-point** IL-1 (endogenous pyrogen) → hypothalamic neurons → COX → **PGE2** → EP3 receptors → temperature set-point raised. Aspirin blocks COX, so it breaks the chain and reduces fever. ## NBE Trap NBE pairs "prostaglandin" with "PGD2" to trap students who know prostaglandins are involved in fever but confuse the subtype. Similarly, option A reverses the aspirin mechanism to test whether students understand that NSAIDs *reduce* rather than increase fever. ## Clinical Pearl In Indian TB wards and sepsis units, fever driven by IL-1 is why patients with active tuberculosis or bacterial infection respond to antipyretics (paracetamol, ibuprofen) but the underlying infection persists — treating fever does not treat the cause. This distinction guides clinical decision-making: suppress fever for comfort, but always investigate and treat the source of endogenous pyrogen release. _Reference: Robbins & Cotran Pathologic Basis of Disease, Ch. 2 (Inflammation); Harrison's Principles of Internal Medicine, Ch. 7 (Fever)_