**Core Concept**
The question tests the understanding of the anatomy of root canals and the challenges associated with their disinfection and filling. Root canals are complex, narrow structures within teeth that contain the pulp and nerves. Instrumentation of these canals is crucial for their disinfection and subsequent filling with gutta-percha.

**Why the Correct Answer is Right**
Instrumentation of root canals involves the use of files and reamers to shape and enlarge the canal. However, if the canal has not been instrumented, it remains in its natural, narrower state. Disinfectants, including irrigants and medicaments, are often too large to penetrate these uninstrumented canals effectively. This is because the diameter of the canal is insufficient to allow the free movement of these substances. As a result, the canal remains contaminated, and the risk of infection and failure of the endodontic treatment increases.

**Why Each Wrong Option is Incorrect**
**Option A:** Microbes are actually a major concern in uninstrumented root canals. They can easily penetrate these narrow canals and cause infection. The question is not asking about the presence of microbes but rather about the difficulty of reaching the canal with disinfectants.

**Option C:** Gutta-percha is used to fill root canals after they have been instrumented and shaped. It is not a substance that is used to disinfect or reach the canal, but rather to fill and seal it.

**Option D:** "None of the above" is incorrect because one of the options (B. Disinfectants) is indeed the correct answer.

**Clinical Pearl / High-Yield Fact**
A key aspect of endodontic treatment is the proper instrumentation of root canals to ensure their disinfection and filling with gutta-percha. This can be challenging, especially in cases where the canal is narrow or curved.

**✓ Correct Answer: B. Disinfectants**