**Question:** In proximal tubule, HCO3 is absorbed with the step of:

A. Active transport
B. Facilitated diffusion
C. Reabsorption
D. Glomerular filtration

**Core Concept:**
The proximal tubule is a crucial part of nephron, which plays a significant role in reabsorbing essential substances from the filtrate to maintain homeostasis. In the case of bicarbonate (HCO3-), it is primarily absorbed through active transport and facilitated diffusion.

**Why the Correct Answer is Right:**
A. Active transport: Bicarbonate ions (HCO3-) are actively transported back into the bloodstream by the sodium-bicarbonate cotransporter (Na+/HCO3- cotransporter or NBCe1). This mechanism is essential for maintaining the acid-base balance and preventing the loss of bicarbonate in the urine.

B. Facilitated diffusion: Bicarbonate ions can also diffuse passively across the basolateral membrane due to the concentration gradient. This process helps in maintaining the transepithelial transport of HCO3-.

**Why Each Wrong Option is Incorrect:**
C. Reabsorption: Reabsorption of HCO3- occurs in the renal tubules, but the correct answer focuses on the specific steps involved in its transport across the proximal tubule membrane.

D. Glomerular filtration: This option is incorrect as glomerular filtration occurs in the nephrons, while the question is about the proximal tubule's role in HCO3- absorption.

**Clinical Pearl:**
Understanding the mechanisms of HCO3- absorption in the proximal tubule is crucial for maintaining the acid-base balance and electrolyte homeostasis in the body. This process involves both active transport and facilitated diffusion, ensuring efficient removal of waste products and maintaining essential ions in the bloodstream.