**Core Concept:**
Respiratory acidosis is a condition where the blood pH becomes acidic due to an excessive amount of carbon dioxide in the blood. This can occur as a result of decreased respiratory rate, increased respiratory workload, or impaired respiratory muscle function, such as in brain trauma. The primary cause of respiratory acidosis is the accumulation of carbon dioxide (CO2), which leads to a decrease in pH (increased acidity) and an increase in pCO2 (partial pressure of carbon dioxide).

**Why the Correct Answer is Right:**
In respiratory acidosis, the primary cause is the accumulation of carbon dioxide, which leads to the following changes:
1. Increased pCO2 (partial pressure of carbon dioxide)
2. Decreased pH (increased acidity)

In this scenario, respiratory acidosis is caused by decreased respiratory rate due to brain trauma, leading to the accumulation of carbon dioxide in the blood.

**Why Each Wrong Option is Incorrect:**
A. Increased bicarbonate (HCO3-) concentration is associated with respiratory alkalosis (high pH, low pCO2), not acidosis.
B. Increased base excess (BE) is associated with alkalosis (high pH), not acidosis.
C. Altered anion gap is a measure of the difference between strong ions and measured anions in the blood and is not directly related to the cause of respiratory acidosis.
D. Increased oxygen saturation (SpO2) is unrelated to the cause of respiratory acidosis and is primarily influenced by oxygenation status.

**Clinical Pearl:**
In cases of respiratory acidosis, a significant decrease in respiratory rate (less than 8-10 breaths per minute) should raise suspicion for the underlying cause, such as brain trauma. Monitoring the patient's pH, pCO2, and bicarbonate levels will confirm the diagnosis and guide therapeutic interventions.