**Core Concept**
Secondary hemorrhage after tonsillectomy is a potentially life-threatening complication that occurs due to bleeding from the tonsillar bed. This can be caused by inadequate hemostasis during surgery, infection, or disruption of the blood clot that formed initially.

**Why the Correct Answer is Right**
The most common cause of secondary hemorrhage is infection, typically caused by a bacterial or viral pathogen that invades the tonsillar bed. This infection can lead to inflammation and degradation of the fibrin clot, resulting in renewed bleeding. The most common pathogens responsible for post-tonsillectomy hemorrhage are *Streptococcus pyogenes*, *Staphylococcus aureus*, and *Haemophilus influenzae*. The infection can also lead to the release of various pro-inflammatory cytokines and enzymes that contribute to the breakdown of the clot.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while dehydration can increase blood viscosity and worsen bleeding, it is not a direct cause of secondary hemorrhage.
**Option B:** This option is incorrect because while trauma to the throat can cause bleeding, it is not a common cause of secondary hemorrhage after tonsillectomy.
**Option C:** This option is incorrect because while anticoagulant medications can increase the risk of bleeding, they are not a direct cause of secondary hemorrhage.

**Clinical Pearl / High-Yield Fact**
It is essential to monitor patients for signs of infection, such as fever, sore throat, or difficulty swallowing, after tonsillectomy, as early detection and treatment can prevent secondary hemorrhage.

**Correct Answer: D. Infection**