**Core Concept**
Chronic mountain sickness (CMS) is a condition that occurs due to prolonged exposure to high-altitude environments, leading to inadequate adaptation of the body to low oxygen levels. This results in symptoms such as fatigue, headaches, and shortness of breath.

**Why the Correct Answer is Right**
Chronic mountain sickness is characterized by an increased red blood cell mass, which is a compensatory mechanism for the low oxygen levels at high altitudes. However, this increased red blood cell mass can lead to increased blood viscosity, which in turn can cause symptoms such as headache, fatigue, and shortness of breath. The increased red blood cell mass is often accompanied by an increase in hemoglobin levels, which is a key diagnostic feature of CMS.

**Why Each Wrong Option is Incorrect**
**Option A:** is not a feature of CMS.
**Option B:** is not a feature of CMS.
**Option C:** is not a feature of CMS.

**Clinical Pearl / High-Yield Fact**
It is essential to note that CMS can be prevented by gradually acclimatizing to high altitudes, which allows the body to adapt to the low oxygen levels. Acclimatization involves ascending to high altitudes at a rate that allows the body to gradually increase red blood cell production and decrease blood viscosity.

**Correct Answer: D**