**Core Concept**
Thrombosed external hemorrhoids are a common condition characterized by the formation of blood clots in the external hemorrhoidal veins, leading to severe pain and swelling. This condition is often associated with a specific clinical presentation and can be differentiated from other anorectal disorders.

**Why the Correct Answer is Right**
Thrombosed external hemorrhoids typically present with a sharp, severe pain that can last for several days, often subsiding within 3-7 days without treatment. The pain is thought to be due to the thrombosis of the external hemorrhoidal veins, which causes inflammation and irritation of the surrounding tissues. The pain is usually worse with bowel movements and sitting, and may be accompanied by swelling and a palpable mass in the anal region.

**Why Each Wrong Option is Incorrect**
**Option A:** Anal fissure typically presents with tearing pain during defecation, not a sharp, severe pain that subsides over several days. In addition, anal fissures are often accompanied by bleeding and a sentinel pile.
**Option B:** Fistula-in-ano is a chronic condition characterized by an abnormal connection between the epithelialized tract of a fistula and the skin or mucosa of the anus, which can cause chronic pain, discharge, and swelling. It does not typically present with acute, self-limiting pain.
**Option D:** Thrombosed internal hemorrhoids are usually asymptomatic and may not cause significant pain, even if they are thrombosed. Internal hemorrhoids are located above the pectinate line and are not as likely to cause severe pain as external hemorrhoids.

**Clinical Pearl / High-Yield Fact**
A key characteristic of thrombosed external hemorrhoids is that they are often associated with a palpable, tender mass in the anal region, which can be mistaken for other conditions such as an abscess or a tumor.

**✓ Correct Answer: C. Thrombosed external hemorrhoids**