**Core Concept**
Fistula formation refers to the development of an abnormal connection between two epithelial-lined structures, such as the skin and a hollow organ. This process often occurs as a result of an infection or trauma, leading to tissue damage and subsequent healing with the formation of a fistulous tract.

**Why the Correct Answer is Right**
The correct answer, **C. 5 days**, is related to the pathophysiology of fistula formation, particularly in the context of perianal abscesses. The formation of a fistula-in-ano is a known complication of anorectal abscesses, which typically occur within 5 days of the onset of symptoms. This timeframe is significant because it represents the period during which the abscess is most likely to rupture and form a fistulous tract, connecting the abscess cavity to the skin or the anal canal.

**Why Each Wrong Option is Incorrect**
**Option A:** 24 hours is too early for fistula formation to occur, as it does not allow sufficient time for the development of an abscess and subsequent tissue damage.

**Option B:** 48 hours is still too early for fistula formation, as the abscess would not have had sufficient time to mature and rupture, leading to the formation of a fistulous tract.

**Option D:** 2 weeks is too late for fistula formation to occur, as it would imply a delay in the healing process, which is not typically observed in the formation of fistulae.

**Clinical Pearl / High-Yield Fact**
It is essential to recognize that fistula formation is a potential complication of anorectal abscesses, and that prompt treatment of the abscess is crucial to prevent the development of a fistulous tract.

**✓ Correct Answer: C. 5 days**