**Core Concept**
Idiopathic brachial neuritis, also known as Parsonage-Turner syndrome, is an inflammatory neuropathy characterized by acute onset muscle weakness, pain, and wasting of the affected limb. This condition is often associated with recent vaccination, trauma, or infection.

**Why the Correct Answer is Right**
The patient's symptoms, including abrupt onset pain, weakness, and loss of muscle contour, are consistent with idiopathic brachial neuritis. This condition is thought to result from inflammation of the brachial plexus, a complex network of nerves that control the muscles of the shoulder and upper arm. The recent tetanus toxoid immunization may have triggered an immune response that led to this inflammation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not match the patient's symptoms. Tetanus infection typically presents with muscle stiffness, spasms, and rigidity, rather than muscle wasting and pain.

**Option B:** This option is incorrect because it is a contraindication for tetanus toxoid immunization, not a cause of the patient's symptoms.

**Option C:** This option is incorrect because it is a possible cause of tetanus infection, but the patient's symptoms do not match those of tetanus.

**Clinical Pearl / High-Yield Fact**
Idiopathic brachial neuritis can occur after recent vaccination or trauma, and is often associated with pain and muscle weakness in the affected limb. It is essential to consider this diagnosis in patients with similar symptoms, especially after recent vaccination.

**Correct Answer:** C. Tetanus infection