**Core Concept**
A 3.5-cm hard lymph node in the submandibular region raises suspicion for head and neck cancer, particularly squamous cell carcinoma, which often presents with occult primary sites. The primary goal is to identify the primary tumor, which can be challenging due to the lack of a palpable mass.

**Why the Correct Answer is Right**
Triple endoscopy (esophagogastroduodenoscopy, laryngoscopy, and bronchoscopy) is the preferred investigation for occult head and neck cancer. This comprehensive approach allows for visualization of the upper aerodigestive tract, including the mucosa of the oral cavity, pharynx, larynx, trachea, and bronchi. It increases the chances of detecting a primary tumor in cases where the primary site is not evident on examination. The high-resolution imaging and direct visualization provided by triple endoscopy facilitate the identification of subtle lesions, which may be missed on routine examination.

**Why Each Wrong Option is Incorrect**
**Option A:** Chest X-ray is not a suitable initial investigation for occult head and neck cancer. While it can provide information on the lung parenchyma, it may not detect primary tumors in the head and neck region, particularly those with occult sites.

**Option C:** Supravital oral mucosa staining (e.g., toluidine blue or methylene blue) can be useful in identifying potentially malignant lesions or tumors in the oral cavity. However, it is not as comprehensive as triple endoscopy, as it does not cover the entire head and neck region.

**Option D:** Laryngoscopy is an essential part of triple endoscopy, but performing it in isolation is not sufficient for a thorough evaluation of occult head and neck cancer.

**Clinical Pearl / High-Yield Fact**
When evaluating a patient with a hard lymph node in the submandibular region, remember to consider the possibility of occult head and neck cancer. Triple endoscopy is a crucial step in the diagnostic workup, as it allows for comprehensive visualization of the upper aerodigestive tract and increases the chances of detecting a primary tumor.

**✓ Correct Answer: B. Triple endoscopy**