**Core Concept**
Rheumatic fever (RF) is a systemic inflammatory disease that occurs after group A beta-hemolytic streptococcal (GABHS) infection, primarily affecting the heart, joints, skin, and brain. Rheumatic heart disease (RHD) is a complication of RF, characterized by valvular damage, particularly affecting the mitral valve.

**Why the Correct Answer is Right**
The correct answer is **a) Caused by group 'A' streptococcus**. Group A streptococcus (GAS) is the primary cause of RF and RHD. The pathogenesis of RF involves molecular mimicry between the M protein of GAS and the host's cardiac tissue, leading to autoimmune responses and inflammation. The M protein of GAS contains epitopes that are similar to the host's cardiac tissue, triggering an immune response that causes damage to the heart valves.

**Why Each Wrong Option is Incorrect**
* **Option B:** Group B streptococcus (GBS) is a common cause of neonatal sepsis and meningitis but is not associated with RF or RHD. GBS does not possess the M protein that triggers the autoimmune response.
* **Option C:** Carriers of GAS have a low risk of developing RF, as the disease typically occurs after an invasive GAS infection, such as pharyngitis or skin infection.
* **Option D:** This statement is incorrect; carriers have a low risk of developing RF.
* **Option E:** While M protein does play a role in the pathogenesis of RF, it is not the M-5 protein that cross-reacts with the host's cardiac tissue; rather, it is the M protein itself.

**Clinical Pearl / High-Yield Fact**
The Jones criteria, developed in 1944, are used to diagnose RF and RHD. The major criteria include carditis, polyarthritis, chorea, erythema marginatum, and subcutaneous nodules, while the minor criteria include fever, arthralgia, and elevated erythrocyte sedimentation rate (ESR).

**Correct Answer:** a) Caused by group 'A' streptococcus