** Shares of card issuer Marqeta fall 39.5% to $3.6

** KeyBanc downgrades MQ to "sector weight" from "overweight" rating, citing substantial revision of 2024 profit forecast and initial 2025 growth to be below Wall Street estimates

** Brokerage says regulatory scrutiny of Marqeta bank partners has created log-jam of new card programs

** 6 of 17 brokerages rate MQ stock "buy" or higher, 11 "hold"; their median PT is $5 - LSEG data

** Including session's moves, stock down ~47.4% YTD

(Reporting by Prakhar Srivastava in Bengaluru)

((Prakhar.srivastava2@thomsonreuters.com))