**Core Concept:** The correct answer relates to the clinical presentation of a neck swelling that moves up with protrusion of the tongue, which is indicative of a lymph node. Lymph nodes are part of the body's immune system and filter out pathogens. Infections (like tuberculosis, syphilis, or HIV) or malignancies can affect lymph nodes, leading to enlargement and mobility.

**Why the Correct Answer is Right:** The correct answer is D: **HIV**. HIV (Human Immunodeficiency Virus) can lead to enlarged and mobile lymph nodes in the neck, as it weakens the immune system and allows for infections to thrive. Infection in the neck lymph nodes is a common early symptom of HIV, known as lymphadenopathy. The swelling moves up when the tongue is protruded due to the proximity of the affected lymph node to the base of the tongue.

**Why Each Wrong Option is Incorrect:**

A. **Tuberculosis:** While tuberculosis can cause lymphadenopathy, the swelling is usually fixed and tender, not mobile. Additionally, tuberculosis typically presents with other symptoms, like fever, cough, and night sweats.

B. **Syphilis:** Similar to tuberculosis, syphilis often causes tender, fixed lymphadenopathy. The clinical presentation typically includes a rash, mucosal lesions, and constitutional symptoms.

C. **Hepatitis:** Hepatitis can cause lymphadenopathy, but the swelling is usually tender and fixed, not mobile. Additionally, hepatitis is often associated with jaundice, fatigue, and loss of appetite.

**Clinical Pearl:** The clinical examination of a patient with a movable neck swelling should be combined with a thorough history, including history of recent infections, travel, or high-risk behaviors. This helps to narrow down the differential diagnosis and guide further investigations and management.

**Correct Answer:** D. HIV.