**Core Concept:**

The question is about the appropriate route of administration for Bacille Calmette-Guérin (BCG), which is an immunotherapy used for the treatment of non-muscle-invasive bladder cancer. BCG is a strain of the Mycobacterium bovis bacteria that stimulates the immune system to target cancerous cells in the bladder.

**Why the Correct Answer is Right:**

BCG is administered via **intratracheal instillation** (inside the airways) as it is a live attenuated strain of the bacteria, and direct instillation into the bladder allows for local activation of the immune system to target the bladder cancer cells. Intratracheal administration ensures that BCG gets aerosolized and reaches the lungs, where it is then inhaled and eventually reaches the bloodstream, allowing for systemic effects as well.

**Why Each Wrong Option is Incorrect:**

1. **A. Intradermal (skin)**: Intradermal administration is incorrect because BCG is a live bacteria, and injecting it into the skin could lead to systemic infection.
2. **B. Intramuscular**: Similar to intradermal administration, intramuscular injection of BCG could cause systemic infection and is not the intended route for local bladder cancer treatment.
3. **C. Intraduodenal**: Intraduodenal administration is incorrect as it involves injecting BCG directly into the small intestine, which would again lead to a systemic infection rather than targeting bladder cancer cells.
4. **D. Intravenous**: Administering BCG intravenously would result in a systemic infection and is not the appropriate route for local bladder cancer treatment.

**Clinical Pearl:**

The correct administration route for BCG therapy is **intratracheal instillation**, ensuring that the drug reaches the lungs and subsequently the bloodstream for systemic effects, while also allowing for local activation of the immune system within the bladder to target cancerous cells. This method allows for a targeted treatment approach for bladder cancer, avoiding the risks associated with systemic administration routes (intradermal, intramuscular, intraduodenal, and intravenous).