**Core Concept**
Diphtheria is a bacterial infection caused by Corynebacterium diphtheriae, which can lead to respiratory and cardiac complications. The identification of asymptomatic carriers is crucial for controlling the spread of the disease. Throat swab culture is a commonly used method for diagnosing diphtheria carriers.

**Why the Correct Answer is Right**
The investigation of choice for diphtheria carriers involves collecting a throat swab specimen. This is because C. diphtheriae can be isolated from the throat of carriers, even if they do not show any symptoms. The bacteria can be cultured on specialized media, such as Tellurite agar, which allows for the identification of the organism. The throat swab culture is considered the gold standard for diagnosing diphtheria carriers.

**Why Each Wrong Option is Incorrect**
**Option A:** Blood culture is not the investigation of choice for diphtheria carriers because C. diphtheriae is primarily a respiratory pathogen, and blood cultures are not typically positive in carriers.
**Option B:** Nasal swab culture may not be as sensitive as throat swab culture for detecting C. diphtheriae in carriers.
**Option C:** PCR (Polymerase Chain Reaction) is a molecular diagnostic technique that can be used to detect C. diphtheriae DNA, but it is not the investigation of choice for routine screening of carriers.

**Clinical Pearl / High-Yield Fact**
It's essential to note that diphtheria carriers can be asymptomatic, and the throat swab culture may remain positive for up to 2 weeks after antibiotic treatment. This highlights the importance of repeated throat swab cultures and close contact tracing to control the spread of the disease.

**Correct Answer:** C. Throat swab culture