**Core Concept**
Ludwig's angina is a severe infection of the submandibular space, characterized by rapid progression of cellulitis and potential airway compromise. This condition is caused by bacterial infection, typically involving anaerobic and aerobic organisms. The infection spreads through the submandibular lymphatic vessels and the submandibular space, leading to severe swelling and potential airway obstruction.

**Why the Correct Answer is Right**
Ludwig's angina typically results from the infection of the submandibular space by bacteria, which can enter through the submandibular gland or the submandibular lymph nodes. The bacteria multiply rapidly, causing inflammation and edema in the submandibular space. This leads to the characteristic symptoms of Ludwig's angina, including severe pain, swelling, and difficulty swallowing or breathing. The infection can be caused by both aerobic and anaerobic bacteria, including Streptococcus, Staphylococcus, and Fusobacterium species.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Ludwig's angina is not caused by infection of the parotid gland, which is located in a different anatomic space.
**Option B:** This option is incorrect because Ludwig's angina is not caused by infection of the sublingual gland, which is a separate salivary gland located under the tongue.
**Option C:** This option is incorrect because Ludwig's angina is not caused by infection of the retropharyngeal space, which is located in the posterior part of the pharynx.

**Clinical Pearl / High-Yield Fact**
Ludwig's angina is a medical emergency that requires prompt treatment to prevent airway compromise and potential mortality. It is essential to recognize the symptoms of Ludwig's angina, including severe pain, swelling, and difficulty swallowing or breathing, and to initiate treatment promptly.

**Correct Answer:** D.