Which of the following is a common symptom of "hardening of the kidney"?

Hypertension is indeed a common symptom associated with "hardening of the kidney," also known as nephrosclerosis. This condition involves the thickening and hardening of the kidney's blood vessels, which can lead to increased resistance to blood flow. As a result, the heart has to work harder to pump blood through the kidneys, contributing to elevated blood pressure. The kidneys play a crucial role in regulating blood pressure through the renin-angiotensin-aldosterone system, and any damage or dysfunction in this system due to nephrosclerosis can exacerbate hypertension.

The other symptoms listed, while potentially present in different kidney conditions or states, do not typically characterize nephrosclerosis. For instance, weight gain may not directly correlate with kidney hardening and can be influenced by various factors unrelated to kidney function. Increased appetite is generally not associated with nephrosclerosis, as other renal diseases may lead to changes in appetite due to toxin buildup. Abdominal pain can occur for numerous reasons but is not a primary symptom associated with nephrosclerosis itself.