**Core Concept:** Antistreptolysin O (ASO) titre is a laboratory test used to measure the immune response to Group A Streptococcus (GAS) infections. It helps in diagnosing streptococcal pharyngitis (strep throat) and scarlet fever, as well as monitoring the treatment response.

**Why the Correct Answer is Right:** ASO titre is typically measured in IU/ml (International Units per milliliter) and increases with the severity of the infection. The test detects the presence of streptococcal antibodies in the bloodstream, indicating past or current GAS infection. The titre usually decreases as the infection resolves and treatment is effective.

**Why Each Wrong Option is Incorrect:**

A. False: ASO titre is not a test for diagnosing streptococcal infections. It indicates the presence of antibodies against GAS, not the infection itself.
B. False: ASO titre does not increase in all cases of streptococcal infections. It is influenced by factors like recent vaccination, sickle cell trait, and rheumatic fever.
C. False: ASO titre does not remain constant over time and can be influenced by the stage of the infection, treatment response, and other factors.
D. False: ASO titre does not remain constant in a healthy individual. It can change with various factors mentioned above.

**Clinical Pearl:** A rising ASO titre indicates an active or recent streptococcal infection, while a falling titre suggests successful treatment. A persistently high titre suggests a chronic infection or autoimmune disease like rheumatic fever. In some cases, a normal ASO titre may be observed due to false negatives in the acute phase or false positives in chronic phase.

**Correct Answer:** A. False: ASO titre is not a test for diagnosing streptococcal infections. It indicates the presence of antibodies against GAS, not the infection itself.