Chloride efflux drives osmotic water loss

Steps:

• Chloride ions exit intestinal cells into the lumen via bacterial toxin effects.
• This increases solute concentration in the intestinal fluid.
• Water moves from cells and blood into the hypertonic intestine to balance osmolarity.
• Excessive water secretion causes severe diarrhea.

Why D is correct:

• Osmosis is the passive movement of water across a semi-permeable membrane from low to high solute concentration, as defined by osmotic pressure gradients.

Why the others are wrong:

• A. Active transport uses energy to move ions against gradients; water loss here is passive.
• B. Active uptake involves energy-dependent absorption into cells; opposite of secretion.
• C. Diffusion applies to solutes like chloride but not specifically to water's bulk movement.

Final answer: D