**Core Concept:**
Atrophic dry nasal mucosa refers to the condition where the nasal mucosa becomes thin and dry due to a decrease in the water-transporting glands and cilia. Encrustations and hardening of the external nose are due to the buildup of salt and bacterial toxins on the dry and damaged mucosa.

**Why the Correct Answer is Right:**
The correct answer is C. Von Ebner's syndrome, also known as Frey's syndrome, is a rare complication of parotidectomy (surgery for parotid gland tumors). It occurs due to the injury or trauma to the sympathetic nerves that supply the lacrimal and salivary glands. As a result, the lacrimal gland, which produces the tear film, becomes overactive, leading to excessive tearing (hyposecretion). At the same time, the parotid gland, which produces saliva, becomes underactive, leading to dry mouth (hyposecretion). This combination of excessive tearing and dry mouth results in the buildup of salt and bacterial toxins on the dry and damaged nasal mucosa, causing encrustations and hardening of the external nose.

**Why Each Wrong Option is Incorrect:**
A. This option is incorrect as it does not explain the specific condition described in the question.
B. This option is incorrect because it does not address the specific symptoms and complications mentioned in the question.
D. This option is incorrect as it does not explain the specific complication resulting from sympathetic nerve injury, unlike the correct answer.

**Clinical Pearl / High-Yield Fact:**
Von Ebner's syndrome, also known as Frey's syndrome, is a clinical entity that should be considered in the differential diagnosis of atrophic dry nasal mucosa and encrustations with hardened external nose in patients who have undergone parotidectomy. A thorough understanding of this condition can help guide appropriate management and improve patient outcomes.