**Core Concept**
Antistreptolysin O (ASLO) titres are a type of serological test used to diagnose and monitor rheumatic fever, a complication of group A beta-haemolytic streptococcal (GABHS) infections. The test measures the levels of antibodies against streptolysin O, a toxin produced by GABHS.

**Why the Correct Answer is Right**
The correct answer is related to the diagnosis of rheumatic fever. During a GABHS infection, the body produces antibodies against the streptococcal toxins, including streptolysin O. ASLO titres are used to detect these antibodies in the blood, indicating a past or current GABHS infection. The test is often used to confirm the diagnosis of rheumatic fever, particularly in patients with symptoms such as fever, joint pain, and a characteristic rash.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because ASLO titres are not used in the diagnosis of scarlet fever, which is another complication of GABHS infections. While ASLO titres can indicate a GABHS infection, they are not specific for scarlet fever.
**Option B:** This option is incorrect because ASLO titres are not used in the diagnosis of post-streptococcal glomerulonephritis (PSGN), a kidney disease that can occur after a GABHS infection. PSGN is typically diagnosed through clinical evaluation and renal function tests.
**Option C:** This option is incorrect because ASLO titres are not used in the diagnosis of streptococcal pharyngitis, which is a throat infection caused by GABHS. While ASLO titres can indicate a GABHS infection, they are not useful for diagnosing streptococcal pharyngitis.

**Clinical Pearl / High-Yield Fact**
It's essential to note that ASLO titres can remain elevated for several weeks after a GABHS infection, making them a useful tool for diagnosing rheumatic fever in patients with a recent history of streptococcal infection.

**Correct Answer:** C.