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Accompanied by his wife Rahat Ara Begum, Fakhrul left Dhaka for Bangkok on a Bangladesh Biman Airlines flight at 12:10pm on Wednesday.
Before the flight, he told bdnews24.com, “I’m going to Bangkok for four days to see a specialist physician.”
He said he had an appointment with a cardiologist at Bumrungrad Hospital at Bangkok.
The BNP leader is scheduled to return home on May 2.
Fakhrul visited Singapore on Jan 28 this year and twice last year for treatment. He also visited the United Sates last year to see doctors.
He was detained for seven months after arrest on Jan 6, 2015.
Diagnoses found a block in the internal carotid artery of the BNP leader.