**Core Concept**
Antistreptolysin 'O' (ASO) titer is a blood test used to diagnose streptococcal infections. It measures the level of antibodies produced in response to streptolysin 'O', a toxin produced by Group A beta-hemolytic streptococci (GABHS). The ASO titer is used to confirm the diagnosis of acute rheumatic fever, a complication of streptococcal infections.

**Why the Correct Answer is Right**
The ASO titer is elevated during the first 2-3 weeks after a streptococcal infection. This is because the body produces antibodies against streptolysin 'O' during this period. The ASO titer is not useful for diagnosing current or recent infections, as it takes time for the antibodies to develop. However, it can be useful for identifying past infections, particularly in cases of suspected rheumatic fever.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the ASO titer is not a direct measure of the severity of the infection. While the titer may be higher in more severe cases, it is not a reliable indicator of disease severity.

**Option B:** This option is incorrect because the ASO titer is not used to diagnose streptococcal pharyngitis. While the titer may be elevated in cases of streptococcal pharyngitis, it is not a specific or sensitive diagnostic test for this condition.

**Option C:** This option is incorrect because the ASO titer is not a useful predictor of the development of rheumatic fever. While a high ASO titer may be associated with an increased risk of rheumatic fever, it is not a reliable predictor of disease.

**Clinical Pearl / High-Yield Fact**
The ASO titer is a useful diagnostic tool, particularly in cases of suspected rheumatic fever. However, it should not be used as the sole diagnostic criterion, as it may be elevated in cases of streptococcal infections that do not lead to rheumatic fever.

**Correct Answer: B. The ASO titer is not used to diagnose streptococcal pharyngitis.**