Correct option is C
EXPLANATION-
Statement A -An antiporter transports two Na⁺ ions and one I⁻ ion across the thyroid follicular cells.
This statement is incorrect.
Typically, the sodium-iodide symporter (NIS) transports 2 Na⁺ ions along with 1 I⁻ ion into the thyroid follicular cells via symport (same direction). An antiporter, by definition, exchanges ions in opposite directions, which is not the case here.
Statement B - Pendrin, a Cl⁻/I⁻ symporter, helps I⁻ entry into the colloid.
This statement is incorrect.
Pendrin is a Cl⁻/I⁻ exchanger (antiporter), not a symporter. It helps move I⁻ into the colloid in exchange for Cl⁻ moving out, i.e., opposite directions.
Statement C - Pendrin, a Cl⁻/I⁻ exchanger, helps I⁻ entry into the colloid.
This statement is correct.
Pendrin acts as an exchanger (antiporter) transporting iodide into the colloid by exchanging it with chloride ions moving out.
Statement D - Iodination of tyrosine residue takes place first on the 3rd position in the thyroglobulin protein.
This statement is correct.
The iodination (addition of iodine) primarily occurs at the 3rd position of the tyrosine residue within the thyroglobulin protein during thyroid hormone synthesis.
So, the correct option is (c) - C and D
