**Core Concept**
C. diphtheriae is a Gram-positive, rod-shaped bacterium responsible for diphtheria, a potentially life-threatening infection characterized by a thick, gray membrane covering the tonsils and throat. Confirmation of the diagnosis is crucial for initiating appropriate treatment and preventing transmission.

**Why the Correct Answer is Right**
The confirmatory test for C. diphtheriae involves demonstrating the presence of the bacterium in a clinical specimen. This can be achieved through isolation of the organism on a selective medium, such as Loeffler's serum slope, which supports the growth of C. diphtheriae while inhibiting other bacteria. The presence of the bacterium is further confirmed by demonstrating its ability to produce the toxin responsible for the disease.

**Why Each Wrong Option is Incorrect**
**Option A:** Culture on blood agar - While blood agar is a general-purpose medium, it does not selectively inhibit other bacteria as effectively as Loeffler's serum slope, making it less suitable for confirming C. diphtheriae.
**Option B:** Gram stain - Although a Gram stain can help identify the Gram-positive nature of C. diphtheriae, it is not a definitive confirmatory test, as many other bacteria may also appear Gram-positive.
**Option C:** PCR - PCR (Polymerase Chain Reaction) can be used to detect the genetic material of C. diphtheriae, but it is not the primary confirmatory test, as it may not distinguish between viable and non-viable organisms.

**Clinical Pearl / High-Yield Fact**
In cases of suspected diphtheria, it is essential to obtain a throat swab specimen from the patient and submit it to the laboratory for culture on Loeffler's serum slope to confirm the presence of C. diphtheriae.

**Correct Answer: C. Isolation on Loeffler's serum slope.**