**Question:** Early sign of tetanus is locked jaw due to early involvement of:

**Core Concept:** Tetanus is a neurological condition caused by the bacterium Clostridium tetani, which produces a powerful neurotoxin called tetanospasmin. This toxin targets the neuromuscular junction, leading to muscle contractions and spasms.

**Why the Correct Answer is Right:** The correct answer is C. Tetanus results from the invasion of C. tetani into the body and its production of tetanospasmin, which affects the neuromuscular junction. When tetanospasmin binds to the presynaptic membrane, it inhibits the release of inhibitory neurotransmitters, such as gamma-aminobutyric acid (GABA), leading to decreased inhibition and increased excitability of the motor neurons.

**Why Each Wrong Option is Incorrect:**

A. Muscle rigidity and spasms are the primary symptoms of tetanus, but the locked jaw (anterolateral synechiae) is a later manifestation, not an early sign.
B. This option refers to the spasmodic pain associated with tetanus, which is unrelated to the early sign of locked jaw.
D. Muscle stiffness and spasms are part of the clinical picture, but the locked jaw is a later manifestation, not an early sign.

**Clinical Pearl:** Locked jaw is a manifestation of tetanus, primarily due to the spasmodic muscle contractions that result from tetanospasmin's action on the neuromuscular junction, leading to decreased inhibitory neurotransmitter release and increased excitability. This early sign alerts healthcare professionals to the possibility of tetanus infection, prompting timely diagnosis and treatment to prevent severe complications and morbidity.