## **Core Concept**
The question tests knowledge of viral classification, specifically focusing on the characteristics of viral genomes and structures. ss-RNA refers to single-stranded RNA, and "un-enveloped" means the virus lacks a lipid envelope. This question requires the test-taker to identify a virus that matches these specific criteria.

## **Why the Correct Answer is Right**
The correct answer, **Picornavirus**, is a family of viruses known for having a single-stranded RNA genome and being non-enveloped. They are small, 20-30 nm in diameter, and their RNA is positive-sense, meaning it can directly serve as mRNA in the host cell. This characteristic makes them capable of causing a wide range of diseases, from the common cold to more severe conditions like poliomyelitis.

## **Why Each Wrong Option is Incorrect**
- **Option A:** If the option provided is a DNA virus or an enveloped RNA virus, it would be incorrect because it does not match the ss-RNA, un-enveloped criteria.
- **Option B:** If this option refers to an enveloped ss-RNA virus, such as Influenza or HIV, it would be incorrect due to the presence of an envelope.
- **Option C:** If this option is another type of RNA virus but enveloped (like Coronavirus) or not ss-RNA, it would not fit the description.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that **Picornaviruses** include several important pathogens, such as Poliovirus, Hepatitis A virus, Rhinoviruses (common cold), and Enteroviruses. Their non-enveloped characteristic makes them relatively resistant to environmental degradation and disinfectants, contributing to their widespread transmission.

## **Correct Answer:** C. Picornavirus