**Core Concept**
The resting tone of the rectum is an important physiological parameter that reflects the intrinsic contractility of the rectal smooth muscle. It is influenced by various factors, including the autonomic nervous system, hormones, and local factors such as rectal distension.

**Why the Correct Answer is Right**
The resting tone of the rectum is primarily maintained by the continuous contraction of the circular smooth muscle layer, which is regulated by the enteric nervous system. This intrinsic contractility is essential for maintaining continence and facilitating defecation. The resting tone is decreased in conditions where there is a loss of enteric nervous system control or damage to the rectal smooth muscle.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it is a condition that affects the rectal smooth muscle, leading to a decrease in resting tone. Hirschsprung's disease is characterized by the absence of enteric nervous system cells in the rectal submucosa, resulting in a failure of the rectal smooth muscle to relax and a decrease in resting tone.
* **Option B:** This option is incorrect because it is a condition that affects the rectal smooth muscle, leading to a decrease in resting tone. Chagas disease is caused by the parasite Trypanosoma cruzi, which can damage the rectal smooth muscle and lead to a decrease in resting tone.
* **Option C:** This option is incorrect because it is a condition that is associated with an increase in resting tone, not a decrease. Spinal cord injury can lead to an increase in resting tone due to the loss of inhibitory input from the spinal cord.

**Clinical Pearl / High-Yield Fact**
The resting tone of the rectum can be affected by various conditions, including Hirschsprung's disease, Chagas disease, and spinal cord injury. It is essential to consider these conditions when evaluating patients with rectal dysfunction.

**Correct Answer: C. Spinal cord injury**