**Question:** Earliest change in high altitude is

A. Reduced oxygen saturation
B. Increased breathing rate
C. Reduced blood pressure
D. Decreased body temperature

**Core Concept:** Adaptation to high altitude involves a series of physiological changes to maintain oxygen homeostasis and prevent hypoxia.

**Why the Correct Answer is Right:** The earliest change observed at high altitude is increased breathing rate (choice B). This is due to the increased demand for oxygen by the body to compensate for the reduced partial pressure of oxygen in the atmosphere. Pulmonary ventilation increases to enhance oxygen uptake and reduce hypoxia.

**Why Each Wrong Option is Incorrect:**

A. Reduced oxygen saturation (choice A) occurs later due to decreased oxygen-rich blood returning to the lungs and decreased oxygen diffusion across the alveolar membrane.
C. Reduced blood pressure (choice C) is less likely to be the earliest change, as it is more related to hypovolemia and essential hypertension.
D. Decreased body temperature (choice D) is a later response involving the hypothalamus and sympathetic nervous system activation, rather than an immediate adaptation to high altitude.

**Clinical Pearl:** The "Red Lines" theory explains the sequential response of the body to high altitude hypoxia:
1. Increased breathing rate (bradycardia and reduced oxygen saturation).
2. Vasoconstriction in the skin and gastrointestinal tract to conserve blood flow.
3. Increased cardiac output to maintain systemic blood flow and oxygen delivery.
4. Hematocrit increase to enhance oxygen carrying capacity.
5. Vasodilation of the central and pulmonary circulation to improve oxygen exchange.

**Correct Answer:** B. Increased breathing rate (bradycardia and reduced oxygen saturation) is the earliest change observed at high altitude, as it helps to enhance oxygen uptake and reduce hypoxia.