**Core Concept**
Tetanus is a bacterial infection caused by Clostridium tetani, characterized by muscle rigidity and spasms due to the production of tetanospasmin toxin. The toxin affects the inhibitory neurons in the central nervous system, leading to uncontrolled muscle contractions.

**Why the Correct Answer is Right**
The most common symptom of tetanus is muscle stiffness or rigidity, which is often described as a "lead pipe" or "board-like" rigidity. This occurs due to the tetanospasmin toxin's effect on the inhibitory neurons, specifically the glycinergic and GABAergic neurons in the spinal cord. The toxin blocks the release of these inhibitory neurotransmitters, leading to an unopposed action of excitatory neurotransmitters, resulting in muscle spasms and rigidity. The characteristic "lockjaw" symptom, where the patient is unable to open their mouth or extend their mandible, is a classic presentation of tetanus.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the symptoms of tetanus.
**Option B:** Although tetanus can cause muscle weakness, it is not the most common symptom.
**Option C:** Muscle spasms are a symptom of tetanus, but they are not the most common or characteristic symptom.

**Clinical Pearl / High-Yield Fact**
Tetanus is often referred to as a "100% fatal disease" if left untreated, but with proper antibiotic and supportive care, the mortality rate can be reduced to less than 10%. Early recognition and treatment are crucial in preventing complications and improving outcomes.

**Correct Answer: C. Muscle stiffness or rigidity.**