**Core Concept**
Teratozoospermia is a condition characterized by the presence of sperm with abnormal morphology in the ejaculate. This condition can significantly affect fertility and is often diagnosed during semen analysis. The term "teratozoospermia" comes from the Greek words "teratos," meaning abnormal or defective, and "zoospermia," meaning containing sperm.

**Why the Correct Answer is Right**
Teratozoospermia is diagnosed when more than 4% of the sperm in the ejaculate have abnormal morphology. This can include various defects such as missing or extra heads, tails, or midpieces. The presence of such morphologically defective sperms can make it challenging for the sperm to fertilize the egg, thereby affecting fertility. In semen analysis, the morphology of sperm is assessed under a microscope by examining the shape and structure of the sperm head, neck, and tail.

**Why Each Wrong Option is Incorrect**
**Option A:** Absence of semen is known as aspermia or anejaculation, which is a different condition from teratozoospermia. Aspermia refers to the complete absence of semen in the ejaculate, whereas teratozoospermia involves the presence of morphologically defective sperms.
**Option B:** Absence of sperm is known as azoospermia, which is another distinct condition from teratozoospermia. Azoospermia refers to the complete absence of sperm in the ejaculate, whereas teratozoospermia involves the presence of abnormal sperms.
**Option C:** All dead sperms in ejaculate is known as necrozoospermia, which is a different condition from teratozoospermia. Necrozoospermia refers to the presence of dead or non-viable sperms in the ejaculate, whereas teratozoospermia involves the presence of morphologically defective sperms.

**Clinical Pearl / High-Yield Fact**
Teratozoospermia can be caused by various factors such as genetic disorders, environmental toxins, infections, and certain medical conditions. It is essential to identify the underlying cause of teratozoospermia to develop an effective treatment plan and improve fertility outcomes.

**✓ Correct Answer: D. Morphologically defective sperms**