diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1d7e9988fc..6baa8f7258 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,8 @@
+## 0.22.3
+
+### Enhancements
+- **`partition_md` Markdown `extensions`**: Optional `extensions` list is passed to `markdown.markdown()`; entries may be registered names (`str`) or `markdown.extensions.Extension` instances. Defaults to `["tables", "fenced_code"]`. Invalid values raise `ValueError`.
+
 ## 0.22.2
 
 - Store routing in ElementMetadata
diff --git a/test_unstructured/partition/test_auto.py b/test_unstructured/partition/test_auto.py
index da54e80466..d7927ccc92 100644
--- a/test_unstructured/partition/test_auto.py
+++ b/test_unstructured/partition/test_auto.py
@@ -6,6 +6,7 @@
 import os
 import pathlib
 import tempfile
+import time
 import warnings
 from importlib import import_module
 from typing import Iterator
@@ -1337,11 +1338,31 @@ def test_auto_partition_passes_user_provided_languages_arg_to_PDF():
 )
 def test_auto_partition_detects_pdf_language_per_element(strategy):
     filename = example_doc_path("language-docs/fr_olap.pdf")
-    elements = partition(
-        filename=filename,
-        strategy=strategy,
-        detect_language_per_element=True,
-    )
+
+    def _partition() -> list[Element]:
+        return partition(
+            filename=filename,
+            strategy=strategy,
+            detect_language_per_element=True,
+        )
+
+    # OCR_ONLY shells out to Tesseract with a temp PNG; under CI load the file can disappear
+    # before Tesseract reads it ("cannot read input file"). Retry a few times on that flake.
+    if strategy == PartitionStrategy.OCR_ONLY:
+        from unstructured_pytesseract import TesseractError
+
+        elements: list[Element] | None = None
+        for attempt in range(3):
+            try:
+                elements = _partition()
+                break
+            except TesseractError as e:
+                if attempt == 2 or "cannot read input file" not in str(e).lower():
+                    raise
+                time.sleep(0.25 * (attempt + 1))
+        assert elements is not None
+    else:
+        elements = _partition()
 
     assert len(elements) > 0
     assert elements[0].metadata.languages == ["fra"]
diff --git a/test_unstructured/partition/test_md.py b/test_unstructured/partition/test_md.py
index bf294dbdb9..6ae8b98872 100644
--- a/test_unstructured/partition/test_md.py
+++ b/test_unstructured/partition/test_md.py
@@ -5,6 +5,7 @@
 
 import pytest
 import requests
+from markdown.extensions.fenced_code import FencedCodeExtension
 from pytest_mock import MockFixture
 
 from test_unstructured.unit_utils import assert_round_trips_through_JSON, example_doc_path
@@ -341,3 +342,53 @@ def test_partition_fenced_code():
     assert elements[3].text == expected_xml
 
     assert elements[4].text == expected_xml
+
+
+def test_partition_md_custom_extensions_parameter():
+    """User can override markdown extensions via `extensions` kwarg (fixes #4006)."""
+    text = """```bash
+# create the container
+docker run -dt --name unstructured downloads.unstructured.io/unstructured-io/unstructured:latest
+```"""
+
+    expected_body = (
+        "# create the container\n"
+        "docker run -dt --name unstructured "
+        "downloads.unstructured.io/unstructured-io/unstructured:latest"
+    )
+
+    # Without fenced_code, ``#`` inside the fence is parsed as a heading (undesired).
+    elements_tables_only = partition_md(text=text, extensions=["tables"])
+    assert any(isinstance(el, Title) for el in elements_tables_only)
+
+    # Default and explicit fenced_code keep the block as one element (CodeSnippet from HTML).
+    assert len(partition_md(text=text)) == 1
+    elements_fenced = partition_md(text=text, extensions=["fenced_code"])
+    assert len(elements_fenced) == 1
+    assert elements_fenced[0].category == ElementType.CODE_SNIPPET
+    assert elements_fenced[0].text == expected_body
+
+    # Extension instances (normal Python-Markdown API) match string extension names.
+    elements_instance = partition_md(text=text, extensions=[FencedCodeExtension()])
+    assert elements_instance == elements_fenced
+
+
+def test_partition_md_extensions_not_list_raises():
+    with pytest.raises(ValueError, match="'extensions' must be a list"):
+        partition_md(text="# Hi", extensions=("tables",))  # type: ignore[arg-type]
+
+
+def test_partition_md_extensions_invalid_item_raises():
+    with pytest.raises(ValueError, match="Each entry in 'extensions'"):
+        partition_md(text="# Hi", extensions=[42])  # type: ignore[list-item]
+
+
+def test_partition_md_tables_only_differs_from_default_for_code_fence():
+    """Without ``fenced_code``, ``#`` inside a fence can become a Title (see #4006)."""
+    text = """```bash
+# line
+```"""
+    default_el = partition_md(text=text)[0]
+    tables_only_els = partition_md(text=text, extensions=["tables"])
+    assert default_el.category == ElementType.CODE_SNIPPET
+    assert any(e.category == ElementType.TITLE for e in tables_only_els)
diff --git a/unstructured/__version__.py b/unstructured/__version__.py
index 31dba6e8f3..f0c78174b4 100644
--- a/unstructured/__version__.py
+++ b/unstructured/__version__.py
@@ -1 +1 @@
-__version__ = "0.22.2"  # pragma: no cover
+__version__ = "0.22.3"  # pragma: no cover
diff --git a/unstructured/partition/md.py b/unstructured/partition/md.py
index 9677d41493..731faf32aa 100644
--- a/unstructured/partition/md.py
+++ b/unstructured/partition/md.py
@@ -4,6 +4,7 @@
 
 import markdown
 import requests
+from markdown.extensions import Extension
 
 from unstructured.documents.elements import Element
 from unstructured.file_utils.encoding import read_txt_file
@@ -21,6 +22,28 @@ def optional_decode(contents: str | bytes) -> str:
 
 DETECTION_ORIGIN: str = "md"
 
+_DEFAULT_MARKDOWN_EXTENSIONS: list[str] = ["tables", "fenced_code"]
+
+
+def _validate_markdown_extensions(extensions: Any) -> list[Any]:
+    """Return ``extensions`` if it is a list of strings and/or ``Extension`` instances.
+
+    Python-Markdown accepts extension entry points as registered names (``str``) or configured
+    ``Extension`` instances; both are supported here. Any other shape raises ``ValueError``.
+    """
+    if not isinstance(extensions, list):
+        raise ValueError(
+            "'extensions' must be a list of extension names (str) and/or "
+            f"markdown.extensions.Extension instances, got {type(extensions).__name__!r}"
+        )
+    for item in extensions:
+        if not isinstance(item, (str, Extension)):
+            raise ValueError(
+                "Each entry in 'extensions' must be a str or markdown.extensions.Extension "
+                f"instance, got {type(item).__name__}: {item!r}"
+            )
+    return extensions
+
 
 def partition_md(
     filename: str | None = None,
@@ -49,6 +72,12 @@ def partition_md(
     languages
         The languages present in the document. Use ``["auto"]`` to detect (default when None).
         Use ``[""]`` to disable language detection.
+
+    Other keyword arguments are forwarded to ``partition_html``. In addition, ``extensions`` may be
+    passed to ``markdown.markdown()`` as a list of registered extension names (``str``) and/or
+    configured ``markdown.extensions.Extension`` instances. The default is
+    ``["tables", "fenced_code"]``. Pass e.g. ``extensions=["tables"]`` if you need the legacy
+    behavior where ``#`` inside unfenced content is parsed as a heading (see #4006).
     """
     if text is None:
         text = ""
@@ -77,7 +106,12 @@ def partition_md(
 
         text = response.text
 
-    html = markdown.markdown(text, extensions=["tables", "fenced_code"])
+    # -- optional markdown extensions; default matches historical partition_md behavior --
+    extensions = _validate_markdown_extensions(
+        kwargs.pop("extensions", _DEFAULT_MARKDOWN_EXTENSIONS)
+    )
+
+    html = markdown.markdown(text, extensions=extensions)
 
     html_kwargs: dict[str, Any] = {
         "text": html,