**Question:** True statement about antistreptolysin 'O' titre is:

A. Antistreptolysin 'O' titre is elevated in streptococcal infections
B. Antistreptolysin 'O' titre is elevated in all bacterial infections
C. Antistreptolysin 'O' titre is elevated in viral infections
D. Antistreptolysin 'O' titre is elevated in autoimmune diseases

**Correct Answer:**

A. **Antistreptolysin 'O' titre is elevated in streptococcal infections:**

The antistreptolysin 'O' titre (ASOT) is a laboratory test used to assess the immune response against Streptococcus pyogenes, a bacterium responsible for causing streptococcal infections. The test measures the presence and titer of specific antibodies in the patient's blood, which are produced in response to streptococcal antigens. The elevated ASOT indicates that the patient has been previously infected by streptococcal bacteria, but it does not specifically diagnose current streptococcal infection.

**Why the Correct Answer is Right:**

1. Streptococcal infections involve the production of streptococcal antigens, which are foreign to the human body.
2. The immune system responds to these antigens by generating specific antibodies, such as ASOT.
3. ASOT is not specific to current infection but indicates previous exposure to Streptococcus pyogenes.

**Why Other Options are Incorrect:**

B. **Antistreptolysin 'O' titre is elevated in all bacterial infections:**
- ASOT is specific to Streptococcus pyogenes infections, not to all bacterial infections.
- Different bacterial infections involve different antigens and antibodies; ASOT is not a general indicator of bacterial infections.

C. **Antistreptolysin 'O' titre is elevated in viral infections:**
- ASOT is a measure of immune response to bacterial antigens, not viral antigens.
- Viral infections involve entirely different immune mechanisms and biomarkers that are not measured by ASOT.

D. **Antistreptolysin 'O' titre is elevated in autoimmune diseases:**
- ASOT is a measure of immune response to bacterial antigens, not autoimmune diseases.
- Autoimmune diseases are characterized by the immune system attacking the patient's own tissues, which is unrelated to ASOT response.

**Clinical Pearl:**

ASOT is a valuable tool in the diagnosis and surveillance of streptococcal infections, particularly when associated with complications like acute rheumatic fever and streptococcal glomerulonephritis. A positive ASOT indicates a higher risk of developing these complications in patients with streptococcal pharyngitis or impetigo. However, a negative test does not exclude the possibility of streptococcal infections or their complications.