Determining the country in South Asia most similar to Central Asia requires considering various social, cultural, economic, and political factors.
One way to answer this complex question is by analyzing the data from the Country Similarity Index. The Index attempts to quantify how similar countries are to each other relative to other countries, using a variety of statistics from five broad categories: demographics, culture, politics, infrastructure, and geography.
The Index suggests that Pakistan is the most similar country in South Asia to Central Asia. However, Pakistan could be considered to be part of the Middle East region as well. It is the crossroads between South Asia, the Middle East, and Central Asia. On the other end of the spectrum, the data suggests that Bhutan is the least similar.
(Central Asian countries defined as Azerbaijan, Kazakhstan, Kyrgyzstan, Mongolia, Tajikistan, Turkey, Turkmenistan, and Uzbekistan)

Culture and geography are the two biggest reasons why Pakistan is the most similar South Asian country to Central Asia. Not only do Pakistan and Central Asia share similar faiths, but they also have similar climates as well.
Culture
Pakistan and most Central Asian countries are predominately Muslim. Due to their religious beliefs, most of their men are circumcised. In addition, pork is not commonly found in their cuisines. Furthermore, the consumption of alcohol tends to not consumed at high rates in these countries. Pakistan’s diet and agriculture is heavily based on wheat, like many Central Asian nations.


Geography
Pakistan’s geography shares some similarities with Central Asia too. Northern Pakistan is characterized by the presence of the Himalayas, Karakoram, and Hindu Kush mountain ranges. These mountain ranges are similar to those found in Central Asia. At high elevations, Pakistan’s climate is comparable to that of Central Asian countries. Both experience cold winters. Furthermore, both areas tend to get little rainfall. Much of their land is not forested.

The Most Similar Central Asian Country to Pakistan
The Country Similarity Index suggests that Tajikistan is the most similar Central Asian country to Pakistan. Their people have somewhat similar ancestry and speak Indo-European languages, unlike much of Central Asia. Like northern Pakistan, Tajikistan is characterized by rugged mountainous terrain, particularly in the Pamir and Alay ranges. Both countries face similar challenges related to economic development, infrastructure, governance, and security.