**Core Concept**
ASO (Anti-Streptolysin O) titre is a blood test used to diagnose streptococcal infections. It measures the level of antibodies produced in response to the Streptococcus pyogenes bacterium. The ASO titre is often used to confirm a diagnosis of acute rheumatic fever (ARF) or post-streptococcal glomerulonephritis (PSGN).

**Why the Correct Answer is Right**
The correct answer is D. ASO titre may not be elevated in 20% of cases of carditis. This is because the ASO titre is not always elevated in patients with carditis, which is a major criterion for diagnosing ARF. Carditis can occur due to an autoimmune response to the streptococcal infection, and the ASO titre may not always correlate with the severity of the carditis. This is because the autoimmune response can be triggered by other mechanisms, such as molecular mimicry, where the immune system mistakenly attacks the body's own tissues.

**Why Each Wrong Option is Incorrect**
**Option A:** ASO titre can indeed be increased in school children, especially during streptococcal pharyngitis outbreaks. This is because school children are more likely to be exposed to streptococcal infections due to close living and playing conditions.
**Option B:** ASO titre may indeed be negative in post-streptococcal glomerulonephritis (PSGN). PSGN is a kidney disease that can occur after a streptococcal infection, and the ASO titre may not always be elevated in these cases.
**Option C:** This is the correct answer. ASO titre is actually included in the minor criteria for diagnosing acute rheumatic fever (ARF), not the major criteria. The Jones criteria are used to diagnose ARF, and they consist of major and minor criteria. The major criteria include carditis, arthritis, chorea, erythema marginatum, and subcutaneous nodules, while the minor criteria include fever, arthralgia, elevated ESR, and elevated ASO titre.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the ASO titre is not always a reliable diagnostic tool, especially in cases of PSGN or carditis. A negative ASO titre does not rule out a streptococcal infection, and a positive ASO titre does not always confirm a diagnosis of ARF.

**Correct Answer Line**
✓ Correct Answer: C. ASO titre is not included in major criteria in Jones criteria.