**Core Concept**
The diagnosis of a white patch over the tonsils in a child is best made by culture, which is a key principle in microbiological diagnosis. This principle involves the isolation and identification of microorganisms from clinical specimens to confirm the presence of an infectious agent. In this context, the white patch is likely to be a result of a bacterial infection.

**Why the Correct Answer is Right**
The white patch over the tonsils is most likely to be a case of tonsillitis, which can be caused by various bacteria such as *Streptococcus pyogenes*. Culture is the gold standard for diagnosing bacterial infections, as it allows for the direct isolation and identification of the causative organism. This is particularly important in cases where antibiotic therapy is being considered, as it helps to guide treatment decisions and prevent antibiotic resistance.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because PCR (polymerase chain reaction) is a molecular diagnostic technique that can detect the presence of bacterial DNA, but it is not as sensitive as culture for detecting viable bacteria.
**Option B:** This option is incorrect because serology involves the detection of antibodies against a specific pathogen, but it does not provide a definitive diagnosis of an active infection.
**Option C:** This option is incorrect because clinical examination alone is not sufficient to diagnose a bacterial infection, as it can be nonspecific and may not accurately distinguish between different types of infections.

**Clinical Pearl / High-Yield Fact**
In cases of suspected bacterial tonsillitis, it is essential to obtain a throat swab culture to confirm the diagnosis and guide antibiotic therapy. This is particularly important in children, as untreated streptococcal infections can lead to complications such as rheumatic fever.

**Correct Answer:** A. Culture is the gold standard for diagnosing bacterial infections, including tonsillitis.