**Core Concept**
The resting pressure in the anal canal is primarily maintained by the internal anal sphincter, which is a smooth muscle component. This pressure is essential for maintaining continence and preventing fecal incontinence. The internal anal sphincter is a part of the involuntary control of defecation.

**Why the Correct Answer is Right**
The resting pressure is generated by the tonic contraction of the internal anal sphincter, which is an involuntary smooth muscle. This contraction is mediated by the enteric nervous system, specifically by the inhibitory neurotransmitter nitric oxide, which causes relaxation of the smooth muscle. The internal anal sphincter is also influenced by the sympathetic nervous system, which can cause contraction of the sphincter. The combination of these two mechanisms helps to maintain the resting pressure in the anal canal.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the external anal sphincter, which is a skeletal muscle, is primarily responsible for voluntary control of defecation, not the resting pressure.
**Option B:** This option is incorrect because the rectal mucosa does not play a significant role in maintaining the resting pressure in the anal canal.
**Option C:** This option is incorrect because the anal valves and pectinate line are anatomical features of the anal canal, but they do not contribute to the resting pressure.

**Clinical Pearl / High-Yield Fact**
The internal anal sphincter is a critical component of the anal canal, and its dysfunction can lead to fecal incontinence. Understanding the physiology of the internal anal sphincter is essential for diagnosing and managing patients with fecal incontinence.

**Correct Answer:** A. The resting pressure in the anal canal is primarily maintained by the internal anal sphincter.