**Question:** All of the following are true about the papovavirus except -

A. Papovavirus is a family of viruses, which are single-stranded, non-enveloped, linear DNA viruses.
B. Papovavirus can cause cancer, including Burkitt's lymphoma and squamous cell carcinoma.
C. Papovavirus has a non-enveloped capsid made of protein.
D. Papovavirus uses direct cell fusion as a method of entry into host cells.

**Core Concept:** Papovavirus is a group of viruses that belong to the family Papovaviridae. These viruses are characterized by their single-stranded, non-enveloped, linear DNA genomes. Papovaviruses can cause cancer, such as Burkitt's lymphoma and squamous cell carcinoma, due to their oncogenic properties.

**Why the Correct Answer is Right:** Option D is incorrect because papovaviruses do not use direct cell fusion as a method of entry into host cells. Instead, they utilize binding proteins on the viral capsid to bind to specific host cell receptors, allowing the virus to enter the cell through endocytosis.

**Why Each Wrong Option is Incorrect:**

A. Papovavirus does have an enveloped capsid, which is covered by a lipid membrane extracted from the host cell during budding, not because it's not enveloped.
B. Papovavirus is not exclusively responsible for cancer. While it can cause cancer, other viruses and genetic factors play a significant role in cancer development as well.
C. Papovavirus has a non-enveloped capsid, but it is made of proteins, not just DNA. The viral capsid is composed of proteins that form a structural scaffold for the viral DNA.

**Clinical Pearl:** Papovavirus-induced cancers are a result of the virus's oncogenic proteins, such as T-antigen, which can disrupt cellular processes and lead to uncontrolled cell growth and the development of tumors.

**Correct Answer:** D (Papovavirus uses direct cell fusion as a method of entry into host cells)

**Core Concepts:** Papovavirus entry into host cells is achieved through endocytosis, a process involving the engulfment of the virus by the host cell membrane. This is distinct from direct cell fusion, which is typically performed by retroviruses and herpesviruses.

**Why Each Wrong Option is Incorrect:**

A. Enveloped capsid refers to the viral capsid being covered by a lipid membrane derived from the host cell during budding. This does not negate the use of endocytosis for viral entry into host cells.
B. While papovavirus-induced cancers are a concern, other viruses (e.g., Epstein-Barr virus) and genetic factors (e.g., ultraviolet radiation) also contribute to cancer development.
C. Papovavirus uses endocytosis for entry into host cells, not direct cell fusion. The viral capsid is composed of proteins, which form a structural scaffold for the viral DNA.