**Core Concept**
In an erect posture, the distribution of ventilation and perfusion in the lungs is not uniform due to the effects of gravity. This leads to differences in the ventilation-perfusion (V/Q) ratio across different parts of the lungs.

**Why the Correct Answer is Right**
The V/Q ratio is a critical parameter in determining the efficiency of gas exchange in the lungs. In an erect posture, the weight of the blood in the pulmonary capillaries causes the perfusion to be greater at the bases of the lungs compared to the apices. However, the ventilation remains relatively uniform across the lungs due to the elasticity of the lungs and the effects of the pleura. As a result, the V/Q ratio is maximum at the apices of the lungs, where perfusion is relatively low and ventilation is relatively high.

**Why Each Wrong Option is Incorrect**
* **Option B:** The V/Q ratio is actually minimum at the bases of the lungs, not maximum, due to the increased perfusion compared to ventilation.
* **Option C:** While ventilation is relatively uniform across the lungs, it is not maximum at the apices. The apices receive less ventilation compared to the bases due to the effects of gravity.
* **Option D:** Perfusion is not maximum at the apices of the lungs. The bases receive more perfusion due to the weight of the blood in the pulmonary capillaries.

**Clinical Pearl / High-Yield Fact**
The V/Q mismatch is a common cause of hypoxemia in patients with lung disease. Understanding the effects of gravity on ventilation and perfusion is critical in managing patients with respiratory problems.

**✓ Correct Answer: A. Ventilation/perfusion ratio is maximum at apex**