**Core Concept**
Upper motor neuron (UMN) syndrome is a clinical condition resulting from damage to the corticospinal tracts or their nuclei in the central nervous system. The key features of UMN syndrome include weakness, spasticity, and an exaggerated deep tendon reflex (DTR) response.

**Why the Correct Answer is Right**
The most important diagnostic sign of UMN syndrome is the presence of an **increased deep tendon reflex (DTR)**, also known as hyperreflexia. This occurs due to the loss of inhibitory control by the upper motor neurons on the alpha motor neurons, which are responsible for muscle contraction. As a result, the reflex arc becomes exaggerated, leading to an overactive response of the muscle to the stimulus. This is a characteristic feature of UMN lesions.

**Why Each Wrong Option is Incorrect**
* **Option A:** Decreased DTR is more characteristic of lower motor neuron lesions, which involve damage to the peripheral nerves or the anterior horn cells.
* **Option B:** Muscle atrophy is a feature of both UMN and lower motor neuron lesions, but it is not the most specific diagnostic sign of UMN syndrome.
* **Option C:** Spasticity is a feature of UMN syndrome, but it is not the most important diagnostic sign. Spasticity can also be seen in other conditions, such as multiple sclerosis or cerebral palsy.

**Clinical Pearl / High-Yield Fact**
Remember the "ABCs" of UMN syndrome: **A**ltered mental status, **B**ilateral weakness, and **C**hanges in DTRs (increased reflexes). This mnemonic can help you remember the key features of UMN syndrome.

**Correct Answer:** A. Increased DTR.