**Core Concept**
Formalin is a fixative that preserves tissue samples by cross-linking proteins, making it impossible to grow microorganisms. This renders the sample unusable for culture. Formalin is commonly used in histopathology for preserving tissue specimens, but it's not suitable for microbiological cultures.

**Why the Correct Answer is Right**
Formalin contains 10% methanol and 40% formaldehyde, which denatures proteins and makes it difficult for microorganisms to survive. When a sample is brought to the laboratory in formalin, the fixative has already begun to preserve the tissue, killing any potential pathogens. This makes it impossible to culture microorganisms from the sample. Formalin's presence is a clear indicator that the sample is not suitable for microbiological examination.

**Why Each Wrong Option is Incorrect**
**Option A:** A sample brought within 2 hr of collection is actually ideal for culture, as the shorter time frame reduces the risk of contamination and preserves the viability of microorganisms.
**Option B:** A sample brought in a sterile plastic container is suitable for culture, as the container has not compromised the integrity of the sample.
**Option D:** Cleaning the collection site may actually reduce the risk of contamination, making it a good practice for collecting samples for culture.

**Clinical Pearl / High-Yield Fact**
When sending samples for microbiological culture, it's essential to ensure that the sample is collected and transported properly to prevent contamination and preserve the viability of microorganisms. This includes using sterile containers, handling the sample gently, and transporting it promptly to the laboratory.

**✓ Correct Answer: C. Sample brought in formalin. This renders the sample unusable for culture due to the presence of the fixative.**