**Question:** Tuberculides are seen in –

A. Lymph nodes
B. Respiratory system
C. Skin
D. Gastrointestinal tract

**Core Concept:** Tuberculides are a type of skin reaction that occurs as a result of skin testing with Mycobacterium tuberculosis, the bacterium causing Tuberculosis (TB). These reactions help diagnose TB infection and assess the immune response to the bacteria.

**Why the Correct Answer is Right:** Tuberculides are produced by the intradermal injection of a tuberculin preparation containing purified protein derivative (PPD) or the Mantoux test. The test stimulates the immune system to produce a response against TB antigens. Tuberculides are typically seen in the form of induration (a hard, raised area) or erythema (redness) at the injection site. The size and duration of the reaction help determine the immune response and the likelihood of TB infection.

**Why Each Wrong Option is Incorrect:**

A. Lymph nodes: Tuberculides are evident at the injection site, not in lymph nodes. The lymph nodes may become enlarged as part of the immune response, but the primary site of reaction is the skin.
B. Respiratory system: Tuberculides are skin reactions, not involving the respiratory system. The respiratory system is primarily affected in pulmonary TB.
C. Skin: As mentioned earlier, tuberculides are skin reactions, not specific to the skin.
D. Gastrointestinal tract: Tuberculides are skin reactions, not related to the gastrointestinal tract involvement. TB primarily affects the lungs and lymphatic system, which leads to tuberculides in the skin.

**Clinical Pearl:** Understanding tuberculides is crucial in diagnosing TB infection and assessing the immune response to the tuberculin test. A positive tuberculin test indicates an increased risk of TB infection, while a negative test indicates a low risk. Assessing tuberculides helps in determining the need for further diagnostic tests and treatment.