**Core Concept**
Intradermal administration involves injecting a substance into the dermal layer of the skin, typically used for allergy testing, vaccination, and immunization. This route is preferred for substances that require a localized immune response.

**Why the Correct Answer is Right**
Tuberculin, used in the Mantoux test for tuberculosis diagnosis, is administered intradermally to induce a localized immune response. Similarly, allergens in skin prick tests are injected intradermally to assess allergic reactions. Vaccines like BCG (Bacillus Calmette-Guérin) are also administered intradermally for immunization against tuberculosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Tetanus toxoid is typically administered intramuscularly, not intradermally, to provide systemic immunity against tetanus.
**Option B:** Influenza vaccine is usually administered intramuscularly or subcutaneously, not intradermally, to induce a systemic immune response.
**Option C:** Diphtheria vaccine is also administered intramuscularly or subcutaneously, not intradermally.

**Clinical Pearl / High-Yield Fact**
Intradermal administration is essential for localized immune responses, such as in allergy testing and tuberculosis diagnosis. However, it's not suitable for substances requiring systemic immunity, like vaccines against tetanus and influenza.

**Correct Answer:** A. Tetanus toxoid is typically administered intramuscularly, not intradermally, to provide systemic immunity against tetanus.