**Core Concept**
Mucoviscidosis, also known as cystic fibrosis, is a genetic disorder affecting the respiratory, digestive, and reproductive systems. It is characterized by the production of thick, sticky mucus that clogs the airways and traps bacteria, leading to recurrent respiratory infections.

**Why the Correct Answer is Right**
Cystic fibrosis is caused by mutations in the CFTR (cystic fibrosis transmembrane conductance regulator) gene, which encodes a chloride channel that regulates the function of sweat glands, respiratory epithelium, and other exocrine glands. The defective CFTR protein leads to the production of abnormal mucus, resulting in the symptoms associated with mucoviscidosis. The correct answer is related to the genetic cause of the disease.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not relate to the genetic cause of mucoviscidosis. While environmental factors may contribute to the severity of the disease, they are not the primary cause.
**Option B:** This option is incorrect because it refers to a different disease altogether. Congenital adrenal hyperplasia is a group of genetic disorders affecting the adrenal glands, unrelated to mucoviscidosis.
**Option C:** This option is incorrect because it is a symptom of mucoviscidosis rather than a cause. Recurrent respiratory infections are a common complication of the disease, but they are not the underlying reason for its development.

**Clinical Pearl / High-Yield Fact**
Cystic fibrosis is a classic example of a genetic disorder with a significant impact on multiple organ systems. The defective CFTR protein affects not only the respiratory system but also the digestive system, pancreas, and reproductive system.

**Correct Answer: D. Cystic fibrosis.**