**Core Concept**
Tender buboes are a clinical feature used in the diagnosis of a specific infectious disease. This symptom is characterized by painful, swollen lymph nodes in the groin, armpits, or neck area. The presence of buboes is a key diagnostic criterion for a particular bacterial infection.

**Why the Correct Answer is Right**
The correct answer is related to the diagnosis of bubonic plague, caused by the bacterium Yersinia pestis. The bacteria infect the lymph nodes, causing them to become swollen and painful. This is due to the multiplication of the bacteria within the lymph nodes, leading to an immune response and subsequent inflammation. The swollen lymph nodes, or buboes, are a distinctive feature of bubonic plague and are used as a diagnostic criterion.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because tender buboes are not a diagnostic feature of cellulitis, which is a bacterial skin infection characterized by redness, swelling, and warmth of the skin.
**Option B:** This option is incorrect because tender buboes are not a diagnostic feature of septic arthritis, which is a joint infection characterized by pain, swelling, and limited mobility of the affected joint.
**Option C:** This option is incorrect because tender buboes are not a diagnostic feature of osteomyelitis, which is a bone infection characterized by pain, swelling, and limited mobility of the affected limb.

**Clinical Pearl / High-Yield Fact**
Tender buboes are a classic sign of bubonic plague, a highly infectious and potentially fatal disease if left untreated. Early recognition and treatment of bubonic plague are critical to prevent complications and improve patient outcomes.

**Correct Answer:** C. Bubonic plague.