**Core Concept**
Anti-Streptolysin O (ASO) titres are used to diagnose acute rheumatic fever (ARF), a complication of group A beta-haemolytic streptococcal (GABHS) infection. The ASO titre measures the level of antibodies produced against streptolysin O, an enzyme produced by GABHS. Elevated ASO titres indicate a recent GABHS infection.

**Why the Correct Answer is Right**
Elevated ASO titres are a diagnostic criterion for acute rheumatic fever, especially in the absence of a positive throat culture or rapid streptococcal antigen test. The ASO titre peaks 2-3 weeks after the onset of symptoms and returns to normal within 3-6 months. The presence of elevated ASO titres is indicative of a recent GABHS infection, which is a prerequisite for the development of acute rheumatic fever.

**Why Each Wrong Option is Incorrect**
* **Option A:** ASO titres are not used to diagnose pharyngitis; throat culture or rapid streptococcal antigen test is preferred for this purpose.
* **Option B:** While ASO titres may be elevated in post-streptococcal glomerulonephritis (PSGN), they are not the primary diagnostic criterion for this condition.
* **Option D:** ASO titres are not used to diagnose streptococcal toxic shock syndrome (STSS); clinical presentation and laboratory findings such as blood cultures and PCR are used for diagnosis.

**Clinical Pearl / High-Yield Fact**
Remember that ASO titres may be falsely elevated in patients with a history of recent vaccination or in those with certain autoimmune disorders. Therefore, a single elevated ASO titre should be interpreted in the context of the patient's clinical presentation and other laboratory findings.

**Correct Answer:** C.