One of the statements below contains a word used incorrectly or inappropriately. Choose the option which has the incorrect or inappropriate usage of the word.

All four sentences contain the word "callus," which refers to a thickened area of skin. We examine each option to identify where the word is used incorrectly.

Option A: [A build-up of the callus is one of the most common causes of discomfort.] uses "callus" correctly. Here, it properly refers to thickened skin causing discomfort.

Option B: [The doctor examined the callus but neglected to treat it.] also uses "callus" correctly. Here, it properly refers to a thickened skin area being examined.

Option C: [When it is not too thick, the callus protects the foot from damage.] uses "callus" correctly as well. Here, it properly describes protective thickened skin.

Option D: [He received fifty lashes on his feet delivered with callus indifference.] uses "callus" incorrectly. "Callus" refers to physical skin thickening, not emotional state. The correct word would be "callous" (meaning emotionally hardened).

Only Option D misuses the word "callus." The correct choice is therefore D.