**Core Concept**
The World Health Organization (WHO) classification of hearing loss is a widely used grading system that categorizes hearing impairment based on pure-tone average (PTA) thresholds. The classification ranges from normal hearing (0-25 dB) to profound hearing loss (90 dB or greater).

**Why the Correct Answer is Right**
A hearing loss of 65 dB falls into the category of moderately severe hearing impairment. According to the WHO classification, a PTA threshold of 41-55 dB is considered severe, while 56-70 dB is classified as moderately severe. In this case, the patient's hearing loss of 65 dB corresponds to the moderately severe category, indicating significant difficulty with speech recognition and understanding in noisy environments.

**Why Each Wrong Option is Incorrect**
**Option A:** Mild hearing loss is characterized by a PTA threshold of 26-40 dB, which is not consistent with a 65 dB hearing loss.
**Option B:** Moderate hearing loss is defined by a PTA threshold of 41-55 dB, which is actually more severe than the given hearing loss.
**Option C:** Severe hearing loss is classified by a PTA threshold of 56-70 dB, but the given hearing loss is slightly less severe.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the WHO classification of hearing loss is based on the average of the pure-tone thresholds at 500, 1000, and 2000 Hz. This grading system helps clinicians and audiologists to communicate effectively and provide appropriate treatment plans for patients with varying degrees of hearing impairment.

**✓ Correct Answer: D. Moderately severe**