**Core Concept**
Antistreptolysin 'O' (ASO) titre is a measure of the body's immune response to group A beta-hemolytic streptococcal infections. It is an indicator of past streptococcal infections and can be used to diagnose post-streptococcal sequelae such as rheumatic fever.

**Why the Correct Answer is Right**
The ASO titre is a measure of the IgG antibodies produced in response to the streptococcal toxin streptolysin O. Elevated ASO titres indicate a recent streptococcal infection, which can trigger autoimmune reactions leading to conditions like rheumatic fever. The ASO titre is not a direct measure of the toxin's presence but rather an indicator of the body's immune response to the toxin.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because ASO titre is not used to diagnose acute streptococcal infections, but rather to detect past infections.
**Option B:** This option is incorrect because ASO titre is not a direct measure of the body's immune response to the streptococcal cell wall, but rather to the toxin streptolysin O.
**Option C:** This option is incorrect because ASO titre is not used to diagnose viral infections, but rather bacterial infections caused by group A streptococci.

**Clinical Pearl / High-Yield Fact**
A high ASO titre can indicate a recent streptococcal infection, which can trigger autoimmune reactions leading to conditions like rheumatic fever. A low ASO titre does not rule out a streptococcal infection, as the body's immune response may be delayed or incomplete.

**Correct Answer:** C. ASO titre is used to diagnose post-streptococcal sequelae such as rheumatic fever.